Drop the devstack-single-node-centos-7 nodeset

OpenDev is preparing to remove centos-7 nodes on March 15[*]. This
change drops one nodeset definition which is the last remaining
reference on DevStack's master branch.

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

Change-Id: Icd487e1012263a9b0bc13b529d31ff2025108adf
(cherry picked from commit 1183167)

Author: fungi

.zuul.yaml

@@ -38,16 +38,6 @@
         nodes:
           - controller
 
-- nodeset:
-    name: devstack-single-node-centos-7
-    nodes:
-      - name: controller
-        label: centos-7
-    groups:
-      - name: tempest
-        nodes:
-          - controller
-
 - nodeset:
     name: devstack-single-node-centos-8-stream
     nodes: