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  referenced by anything in the repo and superseded by GitHub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:
openshift/release#81042

Author: jmarrero

.cci.jenkinsfile

@@ -3,10 +3,7 @@
 
 set -xeuo pipefail
 
-# This is used by our OpenShift Prow job; we use the
-# cosa buildroot container
-# https://github.com/coreos/coreos-assembler/pull/730
-# And using `yum` at all means we can flake on fetching rpm metadata
+# Skip if explicitly requested or if not running as root
 if [ -n "${SKIP_INSTALLDEPS:-}" ] || test "$(id -u)" != 0; then
     exit 0
 fi