UX Bug: don't send notification after overriding subscriber disappearing message timer change

STUB (hotfix)

• in #108 (closed) (disappearing messages), we chose to send subscribers an explanatory notification after overriding a change they make to the disappearing message timer in order to enforce the timer setting chosen by admins
• however, users have reported that this message is confusing, particularly since they didn't always necessarily set the messaging timer on purpose. as it is often one of the first things new users see, and they don't understand it, the message can make them think signalboost is broken.
• so... let's keep the overriding behavior but drop the message