Unwind the fallback admin policy knobs.
Part 1 of removing the TLS version fallback code. Later commits will unwind
other pieces.
The admin policy has expired, so there is no need to keep the code around.
BUG=
621780
Committed:
https://crrev.com/4d0b082a62056eed2affe714b07e1324582f7af9
Cr-Commit-Position: refs/heads/master@{#402310}