Remove CentOS-8 jobs

In place with the upstream deprecation process [1] the upstream mirror
redirectors aren't returning links any more.  Eventually the repos
will move into vault/ probably.

At this point, we can't build centos-8, so remove the jobs.  There is
probably some more cleanup we can do, but this gets the gate going
again.

[1] https://www.centos.org/centos-linux-eol/

Depends-On: https://review.opendev.org/c/opendev/glean/+/827197
Change-Id: If819a035a9f3eae1e5546c1ccc6b23c4e2e91cb5

.zuul.d/jobs.yaml

@@ -23,7 +23,6 @@
         - containerfile/focal-build-succeeds
         - openeuler-minimal/20.03-LTS-SP2-build-succeeds
         # Image based tests
-        - centos/8-build-succeeds
         - centos/8-stream-build-succeeds
         - centos/9-stream-build-succeeds
         - fedora/build-succeeds
@@ -53,7 +52,6 @@
     parent: dib-functests-base
     vars:
       dib_functests:
-        - centos-minimal/8-aarch64-build-succeeds
         - centos-minimal/8-stream-aarch64-build-succeeds
         - openeuler-minimal/20.03-LTS-SP2-build-succeeds
 
@@ -83,17 +81,6 @@
         release: '7'
         mirror: "http://{{ zuul_site_mirror_fqdn }}/centos"
 
-- job:
-    name: dib-nodepool-functional-openstack-centos-8-src
-    description: |
-      Test building and booting a Centos 8 image with Nodepool and OpenStack
-    parent: dib-nodepool-functional-src-base
-    vars:
-      nodepool_diskimage:
-        base_element: centos-minimal
-        release: '8'
-        mirror: "http://{{ zuul_site_mirror_fqdn }}/centos"
-
 - job:
     name: dib-nodepool-functional-openstack-centos-8-stream-src
     description: |

.zuul.d/project.yaml

@@ -15,7 +15,6 @@
         - opendev-buildset-registry
         - nodepool-build-image-siblings
         - dib-nodepool-functional-openstack-centos-7-src
-        - dib-nodepool-functional-openstack-centos-8-src
         - dib-nodepool-functional-openstack-centos-8-stream-src
         - dib-nodepool-functional-openstack-centos-9-stream-src
         - dib-nodepool-functional-openstack-fedora-35-containerfile-src
@@ -46,7 +45,6 @@
         - opendev-buildset-registry
         - nodepool-build-image-siblings
         - dib-nodepool-functional-openstack-centos-7-src
-        - dib-nodepool-functional-openstack-centos-8-src
         - dib-nodepool-functional-openstack-centos-8-stream-src
         - dib-nodepool-functional-openstack-centos-9-stream-src
         - dib-nodepool-functional-openstack-fedora-35-containerfile-src