Remove pubsub storage selector from redis-pubsub

This MR removes the pubsub selector from redis-pubsub as it is removed with gitlab-org/gitlab!133066 (merged) being merged. See gitlab-com/gl-infra/scalability#2539 (closed).

The clean-up was perform prior to the MR's merge so it is safe to proceed with this since the selector was no longer in use as of 19 Oct.