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();