Remove Gitlab::Redis::ClusterSessions post migration

What does this MR do and why?

Remove Gitlab::Redis::ClusterSessions post migration

In gitlab-com/gl-infra/data-access/durability&9 (closed), we're migrating Redis Sessions (Redis sentinel) to a new Redis Cluster Sessions to horizontally scale the CPU.

Once the gprd migration gitlab-com/gl-infra/production#19252 (closed) is done, we can remove the migration helpers, Gitlab::Redis::ClusterSessions and its related feature flags.

Part of cleanup post migration in gitlab-com/gl-infra/data-access/durability/team#34

Second attempt

First attempt: !181631 (merged)

This MR unreverts !184508 (merged) because the first attempt introduced error (incident link):

ERR unknown command 'with_readonly_pipeline', with args beginning with: (redis://redis-cluster-sessions-shard-02-01-db-gstg.c.gitlab-staging-1.internal:6379)

• with_readonly_pipeline is a method only available to MultiStore.
• Since this MR removes the MultiStoreWrapper from Gitlab::Redis::Sessions, there is no with_readonly_pipeline method anymore (from Gitlab::Redis::Wrapper).

This error would have been caught in the previous MR if redis sessions were pointed to the Redis Cluster as described in !184562 (comment 2396645560).

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