Remove tether_enable_select_all_prefix_ranges flag

The tether_enable_select_all_prefix_ranges is added from M2020-12
for quick roll back the feature. Since feature is landed over a year
without any problem, remove the flag now.

Bug: 171269957
Test: atest TetheringTests
Change-Id: I3ef414a47dd0a1f80ea2d378601ccd7c14067318
5 files changed