Remove DeviceConfig

Aconfig is the new way to do things. One of these keys was never used
and the other is an old style flag for a feature launched in UDC, so it
should be safe to clean them both up.

Fixes: 334152006
Test: NotificationManagerServiceTest
Change-Id: Icfb0a8d921821be8f5b72718bd0fd0165eeb8c12