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

modules/openstack_project/files/jenkins_job_builder/config/release.yaml

@@ -1,37 +0,0 @@
-- job:
-    name: common-bump-milestone
-    concurrent: false
-    node: master
-
-    properties:
-      - authorization:
-          openstack-release:
-            - job-build
-          ttx:
-            - job-build
-
-    parameters:
-      - choice:
-          name: PROJECT
-          description: On what project to run?
-          choices:
-            - nova
-            - glance
-            - keystone
-            - horizon
-            - quantum
-            - cinder
-            - ceilometer
-            - heat
-      - choice:
-          name: BRANCH
-          description: The branch to update.
-          choices:
-            - master
-            - milestone-proposed
-      - string:
-          name: VALUE
-          description: 'The milestone value to set for this branch.  E.g., "e4" or "rc1".'
-
-    builders:
-      - shell: "/usr/local/jenkins/slave_scripts/bump-milestone.sh"