Revert "Add git fetch --tags to gerrit-git-prep.sh."

This reverts commit dbe5f35a5a595a982316cfffa978b572c17ada5e.

Git fetch operations are time consuming on our Centos6 slaves. The
additional fetch for the tags is nice to have but not necessary in most
cases. Remove it to reduce time spent doing this on the Centos6 slaves.

Change-Id: If2ca089df660b6dda1fb6381a8ea5a2ef6e5304f
Reviewed-on: https://review.openstack.org/36544
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Approved: Jeremy Stanley <fungi@yuggoth.org>
Tested-by: Jenkins