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-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) {