Remove smsMmsChannel notification channel
Summary
Remove the dead "New Message" notification channel.
This channel was used for a tiny period, probably around 4 years ago, when we thought the only way to support sending MMS on Android was to be the default SMS app, thus we wanted to show users a notification when they received messages while our app was in charge of handling them.
Thankfully, we do not need to be the default SMS app, so this notification channel should be removed.
Test Plan
Before:
Looking at the list of notification streams in the settings for KDE Connect will include a "New Message" channel. Notifications are never posted to this channel.
After:
Users will not see a "New Message" channel. Note that it does show a count of deleted streams, to prevent spam. See documentation for explanation.