[TNU08] Remove the "tethering is on" notification

All carriers discarded the requirement to put up a standing
notification when tethering is on. Thus, remove the "tethering
is on" notification.

Bug: 154438388
Test: atest TetheringTests
Change-Id: Ife3915837b6b7b83d3eaaa84b71b6409ff37b71c
Merged-In: Ife3915837b6b7b83d3eaaa84b71b6409ff37b71c
(cherry picked from commit 0171c07d, aosp/1289107)