Zuul: Remove references to centos-7 job nodes

OpenDev is preparing to remove centos-7 nodes on March 15[*]. This
change drops Zuul configuration references to anything directly or
indirectly relying on that label.

[*] https://lists.openstack.org/archives/list/openstack-discuss@lists.openstack.org/message/A2YIY5L7MVYSQMTVZU3L3OM7GLVVZPLK/

Change-Id: I2d5e177ee3a89e0cedd79e564505e6d41f77b8e4
(cherry picked from commit f6640157df)

.zuul.yaml

@@ -8,8 +8,6 @@
       - check-requirements
     check:
       jobs:
-        - solum-devstack-centos:
-            voting: false
         - solum-devstack
         - solum-tempest-ipv6-only
         - solum-tox-bandit-baseline:
@@ -19,11 +17,6 @@
         - solum-devstack
         - solum-tempest-ipv6-only
 
-- job:
-    name: solum-devstack-centos
-    parent: solum-tempest-base
-    nodeset: devstack-single-node-centos-7
-
 - job:
     name: solum-devstack
     parent: solum-tempest-base