remove tether related helper functions from OffloadUtils.h

None of these are used by anything besides test code,
so we do need to remove the test code as well.

Test: atest, TreeHugger
Signed-off-by: Maciej Żenczykowski <maze@google.com>
Change-Id: I2989dd6cdabdbefe89c730ca6446c27349d4e3e0