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/tests/commands/mod.rs

@@ -18,7 +18,6 @@ mod drop;
 mod each;
 mod echo;
 mod empty;
-mod enter;
 mod error_make;
 mod every;
 #[cfg(not(windows))]
@@ -30,7 +29,6 @@ mod first;
 mod flatten;
 mod for_;
 mod format;
-mod g;
 mod get;
 mod glob;
 mod group_by;
@@ -54,11 +52,9 @@ mod merge;
 mod mkdir;
 mod move_;
 mod mut_;
-mod n;
 mod network;
 mod nu_check;
 mod open;
-mod p;
 mod par_each;
 mod parse;
 mod path;
@@ -84,7 +80,6 @@ mod select;
 mod semicolon;
 mod seq;
 mod seq_char;
-mod shells;
 mod skip;
 mod sort;
 mod sort_by;