zuul: remove /var/lib/git from server

/var/lib/git was used as a Gerrit replication destination. This is now
done on /srv/ssd/gerrit and the repositories have been cleaned up
manually a few months ago when we removed the replication.

Change-Id: Ia7ca383dea207d5283a4e0e35d1629c64ce0b4f5
Merge "zuul: remove /var/lib/git from server" into production