Revert "bbw centos-stream9 - add s390x"

This reverts commit 3900f5f.

Chicken/egg problem - s390x container needed to build galera-4
which is needed to build container. Missing s390x isn't a big problem
- we really only build LTS platforms for s390x.

.github/workflows/bbw_build_container.yml

@@ -101,8 +101,10 @@ jobs:
           - dockerfile: centos.Dockerfile pip.Dockerfile
             image: quay.io/centos/centos:stream9
             tag: centosstream9
-            platforms: linux/amd64, linux/arm64/v8, linux/ppc64le, linux/s390x
             runner: ubuntu-24.04
+            platforms: linux/amd64, linux/arm64/v8, linux/ppc64le
+            # //TEMP chicken/egg problem - could add s390x
+            # but that would need a galera-4 build from on this image
           - dockerfile: opensuse.Dockerfile pip.Dockerfile
             image: opensuse/leap:15.6
             tag: opensuse15