Remove Gitlab::Redis::ClusterSessions post migration

See merge request https://gitlab.com/gitlab-org/gitlab/-/merge_requests/181631



Merged-by: Eugie Limpin <elimpin@gitlab.com>
Approved-by: Daniyal Arshad <darshad@gitlab.com>
Approved-by: Eugie Limpin <elimpin@gitlab.com>
Reviewed-by: Eugie Limpin <elimpin@gitlab.com>
Co-authored-by: Gregorius Marco <gmarco@gitlab.com>