Remove push refs to gerrit feature

This is in preparation to move to separate merger process, which
will not have access to the trigger.  Similar functionality can
be obtained using the new replication feature, and docs have been
updated to that effect.

Change-Id: I9397f7eb8466af464c8e7adb02d0a8d3eff04f9f
Merge "Remove push refs to gerrit feature"