Drop centos 8 stream jobs

CentOS 8 Stream has reached its end of life and the upstream
package repos have been removed. The CentOS nodes from Zuul and
Nodepool are being removed. See the following email announcement:

https: //lists.opendev.org/archives/list/service-announce@lists.opendev.org/thread/YHPRWKWULKJATAJZPFIZCBHS7HZJE5V2/
Change-Id: I572adaa518ffac6113115360a2d8030c7e405b4b

.zuul.d/install-jobs.yaml

@@ -16,11 +16,6 @@
     parent: rally-install-base
     nodeset: ubuntu-jammy
 
-- job:
-    name: rally-install-centos-8s
-    parent: rally-install-base
-    nodeset: centos-8-stream
-
 - job:
     name: rally-install-centos-9s
     parent: rally-install-base

.zuul.d/zuul.yaml

@@ -19,7 +19,6 @@
         - rally-tox-self
         - rally-install-ubuntu-focal
         - rally-install-ubuntu-jammy
-        - rally-install-centos-8s
         - rally-install-centos-9s
         - rally-docker-build
     gate:
@@ -36,7 +35,6 @@
         - rally-tox-self
         - rally-install-ubuntu-focal
         - rally-install-ubuntu-jammy
-        - rally-install-centos-8s
         - rally-install-centos-9s
     post:
       jobs: