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. Also clean up a lingering
reference to swift-dsvm-functional-py3 which no longer exists.

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

Change-Id: Icd487e1012263a9b0bc13b529d31ff2025108adf
(cherry picked from commit bc59eb81a7)

.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-opensuse-15
     nodes:
@@ -654,9 +644,6 @@
             irrelevant-files: &dsvm-irrelevant-files
               - ^.*\.rst$
               - ^doc/.*$
-        - swift-dsvm-functional-py3:
-            voting: false
-            irrelevant-files: *dsvm-irrelevant-files
         - grenade:
             voting: false
             irrelevant-files: