REFACTOR: remove the shell commands (#8415)

Related to #8368.

# Description
as planned in #8311, the `enter`, `shells`, `g`, `n` and `p` commands
have been re-implemented in pure-`nushell` in the standard library.
this PR removes the `rust` implementations of these commands.

- all the "shells" tests have been removed from
`crates/nu-commnand/tests/commands/` in
2cc6a82da6, except for the `exit` command
- `cd` does not use the `shells` feature in its source code anymore =>
that does not change its single-shell behaviour
- all the command implementations have been removed from
`crates/nu-command/src/shells/`, except for `exit.rs` => `mod.rs` has
been modified accordingly
- the `exit` command now does not compute any "shell" related things
- the `--now` option has been removed from `exit`, as it does not serve
any purpose without sub-shells

# User-Facing Changes
users may now not use `enter`, `shells`, `g`, `n` and `p`
now they would have to use the standard library to have access to
equivalent features, thanks to the `dirs.nu` module introduced by @bobhy
in #8368

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- ⚫ `toolkit test`
- ⚫ `toolkit test stdlib`

# After Submitting
the website will have to be regenerated to reflect the removed commands
👍

crates/nu-command/src/filesystem/cd.rs

@@ -1,5 +1,4 @@
 use crate::filesystem::cd_query::query;
-use crate::{get_current_shell, get_shells};
 #[cfg(unix)]
 use libc::gid_t;
 use nu_engine::{current_dir, CallExt};
@@ -164,23 +163,6 @@ impl Command for Cd {
             val: path.clone(),
             span,
         };
-        let cwd = Value::string(cwd.to_string_lossy(), call.head);
-
-        let mut shells = get_shells(engine_state, stack, cwd);
-        let current_shell = get_current_shell(engine_state, stack);
-        shells[current_shell] = path_value.clone();
-
-        stack.add_env_var(
-            "NUSHELL_SHELLS".into(),
-            Value::List {
-                vals: shells,
-                span: call.head,
-            },
-        );
-        stack.add_env_var(
-            "NUSHELL_CURRENT_SHELL".into(),
-            Value::int(current_shell as i64, call.head),
-        );
 
         if let Some(oldpwd) = stack.get_env_var(engine_state, "PWD") {
             stack.add_env_var("OLDPWD".into(), oldpwd)