Drop clone_git function from install_modules.sh.

We no longer use/need this.

Change-Id: I3474ccf8615d5392f4df514b3171e71551927fe3
Reviewed-on: https://review.openstack.org/18908
Reviewed-by: Monty Taylor <mordred@inaugust.com>
Reviewed-by: Clark Boylan <clark.boylan@gmail.com>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins