[CI] Drop CentOS Linux 8 job and nodeset

CentOS Stream 8 (aka CentOS 8 Stream) is the currently
supported runtime platform. [0]

DevStack works with CentOS Stream only now. [1]

The only usage of the nodeset being dropped is handled by the
Depends-On.

[0] https://governance.openstack.org/tc/reference/runtimes/xena.html
[1] https://review.opendev.org/c/openstack/devstack/+/759122

Depends-On: https://review.opendev.org/c/openstack/cinder-tempest-plugin/+/795159
Change-Id: Ic0f696b46dce3dba529b53a8f9de8cda6b913c7b
(cherry picked from commit 81937a230a)

.zuul.yaml

@@ -48,16 +48,6 @@
         nodes:
           - controller
 
-- nodeset:
-    name: devstack-single-node-centos-8
-    nodes:
-      - name: controller
-        label: centos-8
-    groups:
-      - name: tempest
-        nodes:
-          - controller
-
 - nodeset:
     name: devstack-single-node-centos-8-stream
     nodes:
@@ -576,14 +566,6 @@
 # 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-8
-    parent: tempest-full-py3
-    description: Centos 8 platform test
-    nodeset: devstack-single-node-centos-8
-    voting: false
-    timeout: 9000
-
 - job:
     name: devstack-platform-bionic
     parent: tempest-full-py3
@@ -684,7 +666,6 @@
       jobs:
         - devstack
         - devstack-ipv6
-        - devstack-platform-centos-8
         - devstack-platform-bionic
         - devstack-platform-centos-8-stream
         - devstack-async