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.

docs/mkdocs.yml

@@ -82,7 +82,6 @@ nav:
       - CLI:
           - reference/cli/index.md
           - reference/cli/backups.md
-          - reference/cli/config.md
           - reference/cli/facts.md
           - reference/cli/flakes.md
           - reference/cli/flash.md