TetherController - remove a bunch of no longer used RPC support code

This is now dead code - it is reimplemented in Java in the tethering mainline module.

Test:
  git grep 'getTetherOffloadStats|getAndClearTetherOffloadStats|setTetherOffloadInterfaceQuota|addOffloadRule|removeOffloadRule|validateOffloadRule'
  finds nothing

Test: atest, TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: Ic874726f8a99dd686f81bf7b536cd4840e7f7665