Remove several TetherUtil functions which are no longer needed.

Slowly chipping away at TetherUtil to clean up this code.
This CL also adds an admin check to
ConnectivityService.isTetheringSupported to get parity with the
TetherUtil function before removing it.

Change-Id: Ibe7c5c9fb420d57e5458f77dad30e8a1e751a3e2