Remove unused getTetherStats

Before Android R, getTetherStats were used to collect tethering
iface stats of all clients who extends ITetheringStatsProvider,
which typically contains NetdTetheringStatsProvider and
OffloadController.

However, this always returns 0 since Android R, because:
  1. OffloadController implemented NetworkStatsProvider and no
     longer reports stats through getTetherStats.
  2. NetdTetheringStatsProvider always return 0 by design since
     non-offloaded iface tethering stats are already included
     in native iface stats.

This change is a no-op refactoring to remove unused
getTetherStats and update test which is not pratical.
Fix for offloaded tethering stats will be in follow-up CLs.

Test: atest FrameworksNetTests TetheringTests
Bug: 162292214
Change-Id: Icd0717c5c2807ae3bd98626b897e4b148f142815