[DU11]Remove NetworkStats hiddden API dependencies

Remove the implementation of NetdTetheringStatsProvider#getTetherStats
and getNetworkStatsTethering since all callers are migrated to use
INetd#tetherGetStats directly.

Bug: 209738761
Test: run TrafficStatsTest NetworkUsageStatsTest
Change-Id: I69ac951704548c981d1c6ce8238c1d913d96ef62