lock Https-Only Mode (maybe) and remove UI for disabling it

currently we have this (FF130+ does away with disabling it and adds Try HTTPS First - see https://bugzilla.mozilla.org/show_bug.cgi?id=1907517)

HoM

considering how important secure connections are for privacy, I suggest we lock HoM (stable only) in prefs and remove the UI (all channels)

cc @donuts @richard @pierov