Remove the Until cell altogether

For two reasons:
    1. Since this app is so lightweight, it's actually quicker to just
       open it up and disable notifications than it is to think of and
       select a future date to disable them at.
    2. The Today Extension complicates this feature—if notifications are
       silenced from the Today Extension, how long should they be
       silenced for? An "indefinite" feature would have to be added, and
       then reason 1 would be even MORE relevant.