Remove CentOS Stream 8 leftovers

Change-Id: I74bf0eeaf5012728a5f738826b3adfb3ba24c9df

roles/ipa-build-dib-image/tasks/main.yaml

@@ -1,6 +1,6 @@
-- name: Default to CentOS 8
+- name: Default to CentOS Stream 9
   set_fact:
-    image_release: 8
+    image_release: 9-stream
   when:
     - image_distro in ["centos-minimal", "centos"]
     - not image_release

zuul.d/ironic-python-agent-builder-jobs.yaml

@@ -19,18 +19,6 @@
     vars:
       image_type: 'tinyipa'
 
-# TODO(dtantsur): remove when no other projects use it
-- job:
-    name: ironic-python-agent-build-image-dib-centos8
-    parent: ironic-python-agent-build-image-base
-    required-projects:
-      - openstack/diskimage-builder
-    vars:
-      image_type: 'dib'
-      image_distro: 'centos'
-      image_release: '8-stream'
-      image_target_name: 'centos8'
-
 - job:
     name: ironic-python-agent-build-image-dib-centos9
     parent: ironic-python-agent-build-image-base
@@ -80,18 +68,6 @@
     vars:
       image_type: 'tinyipa'
 
-# TODO(dtantsur): remove when no other projects use it
-- job:
-    name: ironic-python-agent-check-image-dib-centos8
-    parent: ironic-python-agent-check-image-base
-    required-projects:
-      - openstack/diskimage-builder
-    vars:
-      image_type: 'dib'
-      image_distro: 'centos'
-      image_release: '8-stream'
-      image_target_name: 'centos8'
-
 - job:
     name: ironic-python-agent-check-image-dib-centos9
     parent: ironic-python-agent-check-image-base