main: Fix or remove diagnose command

The --diagnose command line option is broken, probably for a while since no one really uses it.

fbx@freedombox:/$ sudo -u plinth /freedombox/run --diagnose
    INFO axes.watch_login     AXES: BEGIN LOG
    INFO axes.watch_login     AXES: Using django-axes version 5.4.3
    INFO axes.watch_login     AXES: blocking by IP only.
    INFO plinth.__main__      FreedomBox Service (Plinth) version - 21.4
    INFO plinth.__main__      Script prefix - /plinth
    INFO plinth.module_loader Initializing apps - help, cockpit, diagnostics, firewall, names, avahi, power, users, apache, security, sso, ssh, upgrades, dynamicdns, storage, backups, snapshot, config, networks, first_boot, datetime, letsencrypt, minetest, searx, wireguard, ejabberd, syncthing, openvpn, shadowsocks, bepasty, monkeysphere, ttrss, transmission, mldonkey, radicale, api, mumble, mediawiki, coturn, matrixsynapse, minidlna, tor, i2p, quassel, zoph, sharing, samba, roundcube, ikiwiki, gitweb, deluge, calibre, infinoted, pagekite, privoxy, jsxc, performance, bind
    INFO plinth.modules.names Added domain freedombox.local of type domain-type-local with services __all__
    INFO plinth.actions       # dynamicdns status
    INFO plinth.actions       $ ikiwiki get-sites
    INFO plinth.modules.letsencrypt Checking if any Let's Encrypt certificates got renewed.
    INFO plinth.actions       # letsencrypt get-status
Traceback (most recent call last):
  File "/freedombox/run", line 8, in <module>
    plinth.__main__.main()
  File "/freedombox/plinth/__main__.py", line 164, in main
    run_diagnostics_and_exit()
  File "/freedombox/plinth/__main__.py", line 89, in run_diagnostics_and_exit
    module = importlib.import_module('plinth.modules.diagnostics.diagnostics')
  File "/usr/lib/python3.9/importlib/__init__.py", line 127, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1030, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1007, in _find_and_load
  File "<frozen importlib._bootstrap>", line 984, in _find_and_load_unlocked
ModuleNotFoundError: No module named 'plinth.modules.diagnostics.diagnostics'

IMO, there isn't much need to keep this option, so it could be removed.

We will just need to update this freedom-maker test that uses it:

https://salsa.debian.org/freedombox-team/freedom-maker/-/blob/master/freedommaker/tests/test_virtualbox.py#L114