Remove the unused ITetheringStatsProvider.

This is safe because :
• There are no longer any existing user of this class in AOSP or
  downstream
• Known previous uses by OEMs/vendors have been removed
• This class is now superseded by the better and more generic
  network stats provider, so that remaining uses if any can be
  retrofitted into this newer, better class

Test: TH
Bug: 64955351
(cherry picked from commit e0ed0d898e926d6cb24153c4aee6a24186a7026b)
(cherry picked from commit cedc5de16ed39d09c9d331cbc9ee475f59637f7c)
(cherry picked from https://googleplex-android-review.googlesource.com/q/commit:6709e62a06331e743824b3586ef52abe221c775f)
Merged-In: I88fee389a3c768baf7c29096d2245734d9dd4fe8
Change-Id: I88fee389a3c768baf7c29096d2245734d9dd4fe8