remove the $nothing variable (#10567)

related to 
- https://github.com/nushell/nushell/pull/10478

# Description
this PR is the followup removal to
https://github.com/nushell/nushell/pull/10478.

# User-Facing Changes
`$nothing` is now an undefined variable, unless define by the user.
```nushell
> $nothing
Error: nu::parser::variable_not_found

  × Variable not found.
   ╭─[entry #1:1:1]
 1 │ $nothing
   · ────┬───
   ·     ╰── variable not found.
   ╰────
```

# Tests + Formatting

# After Submitting
mention that in release notes

crates/nu-cli/src/completions/variable_completions.rs

@@ -43,7 +43,7 @@ impl Completer for VariableCompletion {
         options: &CompletionOptions,
     ) -> Vec<Suggestion> {
         let mut output = vec![];
-        let builtins = ["$nu", "$in", "$env", "$nothing"];
+        let builtins = ["$nu", "$in", "$env"];
         let var_str = std::str::from_utf8(&self.var_context.0)
             .unwrap_or("")
             .to_lowercase();

crates/nu-engine/src/eval.rs

@@ -6,8 +6,8 @@ use nu_protocol::{
         Expression, Math, Operator, PathMember, PipelineElement, Redirection,
     },
     engine::{EngineState, Stack},
-    report_error_new, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Range, Record,
-    ShellError, Span, Spanned, Unit, Value, VarId, ENV_VARIABLE_ID,
+    IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Range, Record, ShellError, Span,
+    Spanned, Unit, Value, VarId, ENV_VARIABLE_ID,
 };
 use std::collections::HashMap;
 
@@ -1128,20 +1128,6 @@ pub fn eval_variable(
     span: Span,
 ) -> Result<Value, ShellError> {
     match var_id {
-        // $nothing
-        nu_protocol::NOTHING_VARIABLE_ID => {
-            report_error_new(
-                engine_state,
-                &ShellError::GenericError(
-                    "Deprecated variable".into(),
-                    "`$nothing` is deprecated and will be removed in 0.87.".into(),
-                    Some(span),
-                    Some("Use `null` instead".into()),
-                    vec![],
-                ),
-            );
-            Ok(Value::nothing(span))
-        }
         // $nu
         nu_protocol::NU_VARIABLE_ID => {
             if let Some(val) = engine_state.get_constant(var_id) {

crates/nu-parser/src/parse_keywords.rs

@@ -3043,7 +3043,7 @@ pub fn parse_let(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipeline
                             .trim_start_matches('$')
                             .to_string();
 
-                    if ["in", "nu", "env", "nothing"].contains(&var_name.as_str()) {
+                    if ["in", "nu", "env"].contains(&var_name.as_str()) {
                         working_set.error(ParseError::NameIsBuiltinVar(var_name, lvalue.span))
                     }
 
@@ -3151,7 +3151,7 @@ pub fn parse_const(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipelin
                             .to_string();
 
                     // TODO: Remove the hard-coded variables, too error-prone
-                    if ["in", "nu", "env", "nothing"].contains(&var_name.as_str()) {
+                    if ["in", "nu", "env"].contains(&var_name.as_str()) {
                         working_set.error(ParseError::NameIsBuiltinVar(var_name, lvalue.span))
                     }
 
@@ -3292,7 +3292,7 @@ pub fn parse_mut(working_set: &mut StateWorkingSet, spans: &[Span]) -> Pipeline
                             .trim_start_matches('$')
                             .to_string();
 
-                    if ["in", "nu", "env", "nothing"].contains(&var_name.as_str()) {
+                    if ["in", "nu", "env"].contains(&var_name.as_str()) {
                         working_set.error(ParseError::NameIsBuiltinVar(var_name, lvalue.span))
                     }
 

crates/nu-parser/src/parser.rs

@@ -18,7 +18,7 @@ use nu_protocol::{
     engine::StateWorkingSet,
     eval_const::{eval_constant, value_as_string},
     span, BlockId, DidYouMean, Flag, ParseError, PositionalArg, Signature, Span, Spanned,
-    SyntaxShape, Type, Unit, VarId, IN_VARIABLE_ID, NOTHING_VARIABLE_ID,
+    SyntaxShape, Type, Unit, VarId, ENV_VARIABLE_ID, IN_VARIABLE_ID,
 };
 
 use crate::parse_keywords::{
@@ -1784,14 +1784,7 @@ pub fn parse_string_interpolation(working_set: &mut StateWorkingSet, span: Span)
 pub fn parse_variable_expr(working_set: &mut StateWorkingSet, span: Span) -> Expression {
     let contents = working_set.get_span_contents(span);
 
-    if contents == b"$nothing" {
-        return Expression {
-            expr: Expr::Var(nu_protocol::NOTHING_VARIABLE_ID),
-            span,
-            ty: Type::Nothing,
-            custom_completion: None,
-        };
-    } else if contents == b"$nu" {
+    if contents == b"$nu" {
         return Expression {
             expr: Expr::Var(nu_protocol::NU_VARIABLE_ID),
             span,
@@ -5834,9 +5827,7 @@ pub fn discover_captures_in_expr(
             discover_captures_in_expr(working_set, expr, seen, seen_blocks, output)?;
         }
         Expr::Var(var_id) => {
-            if (*var_id > NOTHING_VARIABLE_ID || *var_id == IN_VARIABLE_ID)
-                && !seen.contains(var_id)
-            {
+            if (*var_id > ENV_VARIABLE_ID || *var_id == IN_VARIABLE_ID) && !seen.contains(var_id) {
                 output.push((*var_id, expr.span));
             }
         }

crates/nu-protocol/src/engine/engine_state.rs

@@ -113,7 +113,6 @@ const REGEX_CACHE_SIZE: usize = 100; // must be nonzero, otherwise will panic
 pub const NU_VARIABLE_ID: usize = 0;
 pub const IN_VARIABLE_ID: usize = 1;
 pub const ENV_VARIABLE_ID: usize = 2;
-pub const NOTHING_VARIABLE_ID: usize = 3;
 // NOTE: If you add more to this list, make sure to update the > checks based on the last in the list
 
 impl EngineState {

crates/nu-protocol/src/lib.rs

@@ -27,7 +27,7 @@ pub use alias::*;
 pub use cli_error::*;
 pub use config::*;
 pub use did_you_mean::did_you_mean;
-pub use engine::{ENV_VARIABLE_ID, IN_VARIABLE_ID, NOTHING_VARIABLE_ID, NU_VARIABLE_ID};
+pub use engine::{ENV_VARIABLE_ID, IN_VARIABLE_ID, NU_VARIABLE_ID};
 pub use example::*;
 pub use exportable::*;
 pub use id::*;