clan: remove config subcommand

This removes the `config` subcommand from the cli and the documentation
to keep the api surface small.

While this functionality was convenient it doesn't need to be surfaced
by the clan cli.

The remaining `config` python module should be ported to the `clan-app`
in a follow up pr. Because the functionality is currently only used by
the `clan-app`.
Ideally together with: #1830.

pkgs/clan-cli/clan_cli/config/schema.py

@@ -11,6 +11,8 @@ from clan_cli.errors import ClanError, ClanHttpError
 from clan_cli.nix import nix_eval
 
 
+# TODO: When moving the api to `clan-app`, the whole config module should be
+# ported to the `clan-app`, because it is not used by the cli at all.
 @API.register
 def machine_schema(
     flake_dir: Path,