Disable Gerrit auto-abandon

With the advent of ACL-controlled abandon and restore capability in
more recent Gerrit releases, we can now put this power in the hands
of core reviewers for individual projects and should no longer need
to rely on hacky system-wide automated rules to abandon stale
changes.

Change-Id: I85c8ebc548df8328437615bca472eddd4fc0375f
Merge "Disable Gerrit auto-abandon"