Remove pubsub migration helper for actioncable

What does this MR do and why?

This MR removes Gitlab::Redis::Pubsub which was introduced in !126451 (merged) to migrate actioncable workload from the Redis in config/cable.yml to the Redis instance in the production.pubsub key in config/redis.yml.

We can clean up this code as the migration as been done.

See gitlab-com/gl-infra/scalability#2539 (closed)

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.