In https://gitlab.com/gitlab-org/gitlab/-/merge_requests/176108, we
added a new implementation of Redis session store
`Gitlab::Sessions::CacheStore` which is based on
`ActionDispatch::Session::CacheStore` in order to support migration to
Redis Cluster for .com. The whole migration plan can be found in
https://gitlab.com/gitlab-com/gl-infra/data-access/durability/team/-/issues/35.

This removes the old `Gitlab::Sessions::RedisStore` as the redis session
store and the environment variable
`USE_REDIS_CACHE_STORE_AS_SESSION_STORE` used to toggle the initializer
to use the new store.

Self-managed instances wouldn't be affected since we have
`Gitlab::Sessions::CacheStoreCoder` to ensure compatibility within the
two stores. More context at https://gitlab.com/gitlab-com/gl-infra/data-access/durability/team/-/issues/35#note_2283399167

Changelog: other