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• This class is now superseded by the better and more generic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
Merged-In: I88fee389a3c768baf7c29096d2245734d9dd4fe8
Change-Id: I88fee389a3c768baf7c29096d2245734d9dd4fe8