eliminate tethering stuff from bpf_shared.h

These definitions are actually in the Tethering mainline module.
Netd should not depend on these, since they may change as part
of mainline module updates.
Additionally netd no longer has the selinux privs to access
the relevant programs/maps.

Test: atest, TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I26b58953f9b1185f8272f4e53f27e24a388c4f1c