ART: Remove zygote OTA code

The renaming and relabeling of the A/B artifacts is now done in
a special-purpose script started by init, so as to not complicate
the logic here (e.g., name of the current slot).

Partially reverts commit 754479f7.

Bug: 25612095
Bug: 28069686
Change-Id: I4d965361000fd448e199c9af059b6d18bfe928bb