ci: Remove Prow and coreos-ci Jenkins infrastructure

ci: Remove Prow and coreos-ci Jenkins infrastructure

Remove the legacy CI artifacts that are no longer in use:

- ci/prow/: No-op placeholder Dockerfile and script that were kept
  while waiting for the job to be removed from openshift/release.
  Remove them entirely so Prow stops running on every PR.
- .cci.jenkinsfile: Orphaned coreos-ci Jenkins pipeline. Not
  referenced by anything in the repo and superseded by GitHub
  Actions and Konflux.
- ci/installdeps.sh: Remove stale comment referencing the Prow job.

The Prow job was already removed from openshift/release:
https://github.com/openshift/release/pull/81042