Drop centos7 platform job

The centos7 job is running with python2, which is no longer supported by
nova, so we can drop it in master.

Change-Id: Id9ef507dd6f4226d65c6ed3043666b0aa6a3bd1c

Author: jharbott

.zuul.yaml

@@ -520,13 +520,6 @@
 # we often have to rush things through devstack to stabilise the gate,
 # and these platforms don't have the round-the-clock support to avoid
 # becoming blockers in that situation.
-- job:
-    name: devstack-platform-centos-7
-    parent: tempest-full
-    description: Centos 7 platform test
-    nodeset: devstack-single-node-centos-7
-    voting: false
-
 - job:
     name: devstack-platform-opensuse-15
     parent: tempest-full-py3
@@ -621,7 +614,6 @@
         - devstack
         - devstack-xenial
         - devstack-ipv6
-        - devstack-platform-centos-7
         - devstack-platform-opensuse-15
         - devstack-platform-fedora-latest
         - devstack-platform-xenial