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
Change-Id: I1e87206a7f261e587cf079cb6b7025ca16aa4fe4