Remove centos8 wallaby jobs from zuul layouts

This prevents some of the remaining c8 check and gate jobs from
running for stable/wallaby.

We are removing wallaby/c8 integration and component lines in [1]
and keeping only mixed rhel and ffu jobs with container builds as
part of the existing wallaby/centos9 line. Thus in this patch we
don't alter the wallaby/8 content provider and upgrade jobs as we
want these to continue runing. See [2] for other related patches.

[1] https://review.rdoproject.org/r/q/topic:oooci_teardown_8_wallaby
[2] https://review.opendev.org/q/topic:oooci_teardown_8_wallaby

Change-Id: Ife23e183a279417e393ee9b054e28a63b3f1298e

zuul.d/build-image.yaml

@@ -260,7 +260,7 @@
 - job:
     name: tripleo-buildimage-ironic-python-agent-centos-8
     parent: tripleo-build-images-base-centos-8
-    branches: ^(stable/(train|wallaby)).*$
+    branches: stable/train
     vars:
       tripleo_image_type: ironic-python-agent
     irrelevant-files:
@@ -270,7 +270,7 @@
 - job:
     name: tripleo-buildimage-overcloud-full-centos-8
     parent: tripleo-build-images-base-centos-8
-    branches: ^(stable/(train|wallaby)).*$
+    branches: stable/train
     vars:
       tripleo_image_type: overcloud-full
     irrelevant-files:

zuul.d/standalone-jobs.yaml

@@ -57,7 +57,7 @@
 - job:
     name: tripleo-ci-centos-8-standalone
     parent: tripleo-ci-base-standalone-centos-8
-    branches: ^(stable/(train|wallaby)).*$
+    branches: stable/train
     vars:
       featureset: '052'
     irrelevant-files: &irrelevant_files