Remove the common-bump-milestone job.

We've removed the milestone-branch based infrastructure. This job is no longer
needed.

Change-Id: I5164e312cfb96e6043bbcc4ca9706530a7092b37
Reviewed-on: https://review.openstack.org/20841
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Approved: Monty Taylor <mordred@inaugust.com>
Tested-by: Jenkins