Remove Zuul ref replication.

This reverts commit 87650fa736fb61c6b305f9ea8c3907e4f0168cd9.

The problem of distributing the load of serving Zuul Git refs will
be addressed by separating the merger component so that it can be
scaled out.  Remove this feature which is hopefully new enough that
no one is using it.

Change-Id: Id2be95c85f5c3464a66537ae3095024a964ee1c0
Merge "Remove Zuul ref replication."