OffloadUtils - drop support for tethering tc & xdp program attachment

This also allows removal of PRIO_TETHER4 / PRIO_TETHER6 constants,
and the DOWNSTREAM & UPSTREAM documentation booleans.

PRIO_CLAT can move out of the .h and into the .cpp and we can bump
it by one to make room for future improvements.

Test: atest, TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I236b6c5ee36a9ca7dbf9d30ae6abedac0a2270de