Remove PLATFORM_VERSION_FUTURE_CODENAMES

It has been wrong to split ALL_VERSIONS into exclusive two sets of
before/after TARGET_PLATFORM_VERSION.

And PLATFORM_VERSION_ALL_CODENAMES supports all *active* list of
non-finalized codenames.

Bug: 152960049
Test: m
Exempt-From-Owner-Approval: cp from master
Merged-In: I78ca88758998e440bea72ba2d56d90eea3ec99ae
Change-Id: I78ca88758998e440bea72ba2d56d90eea3ec99ae
(cherry picked from commit 424175d7)