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-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 {