Remove obsolete omnibus-gitlab OS images
What does this MR do and why?
Starting with omnibus-gitlab!6510 (merged), omnibus-gitlab no longer uses images from this repo. Let's remove the unwanted CI jobs and Dockerfiles. (The existing images aren't purged, so older stable branches continue to work).
Related to gitlab-org/distribution/team-tasks#732 (closed)
Checklist
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(If applicable) Add patches if required for upgrading Ruby version in https://gitlab.com/gitlab-org/gitlab-build-images/-/tree/master/patches/ruby.
Edited by Balasankar 'Balu' C