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-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));
             }
         }