Remove Gitlab::Redis::ClusterSessions post migration

In https://gitlab.com/groups/gitlab-com/gl-infra/data-access/durability/-/epics/9, we're migrating Redis Sessions (Redis sentinel) to a new Redis Cluster Sessions to horizontally scale the CPU.
Once the gprd migration https://gitlab.com/gitlab-com/gl-infra/production/-/issues/19252 is done, we can remove the migration helpers, Gitlab::Redis::ClusterSessions and its related feature flags.
Part of cleanup post migration in https://gitlab.com/gitlab-com/gl-infra/data-access/durability/team/-/issues/34

Changelog: other

config/feature_flags/gitlab_com_derisk/use_primary_and_secondary_stores_for_sessions.yml

----
-name: use_primary_and_secondary_stores_for_sessions
-feature_issue_url: https://gitlab.com/gitlab-com/gl-infra/data-access/durability/team/-/issues/28
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175735
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/509337
-milestone: '17.9'
-group: group::durability
-type: gitlab_com_derisk
-default_enabled: false

config/feature_flags/gitlab_com_derisk/use_primary_store_as_default_for_sessions.yml

----
-name: use_primary_store_as_default_for_sessions
-feature_issue_url: https://gitlab.com/gitlab-com/gl-infra/data-access/durability/team/-/issues/28
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/175735
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/509338
-milestone: '17.9'
-group: group::durability
-type: gitlab_com_derisk
-default_enabled: false

lib/gitlab/redis.rb

@@ -10,7 +10,6 @@ module Redis
     ALL_CLASSES = [
       Gitlab::Redis::BufferedCounter,
       Gitlab::Redis::Cache,
-      Gitlab::Redis::ClusterSessions,
       Gitlab::Redis::DbLoadBalancing,
       Gitlab::Redis::FeatureFlag,
       *Gitlab::Redis::Queues.instances.values, # dynamically adds QueueShard* classes

lib/gitlab/redis/cluster_sessions.rb

-# frozen_string_literal: true
-
-module Gitlab
-  module Redis
-    class ClusterSessions < ::Gitlab::Redis::Wrapper
-      class << self
-        # The data we store on Sessions used to be stored on SharedState.
-        def config_fallback
-          SharedState
-        end
-      end
-    end
-  end
-end

lib/gitlab/redis/sessions.rb

@@ -2,7 +2,7 @@
 
 module Gitlab
   module Redis
-    class Sessions < ::Gitlab::Redis::MultiStoreWrapper
+    class Sessions < ::Gitlab::Redis::Wrapper
       SESSION_NAMESPACE = 'session:gitlab'
       USER_SESSIONS_NAMESPACE = 'session:user:gitlab'
       USER_SESSIONS_LOOKUP_NAMESPACE = 'session:lookup:user:gitlab'
@@ -13,10 +13,6 @@ class Sessions < ::Gitlab::Redis::MultiStoreWrapper
       def self.config_fallback
         SharedState
       end
-
-      def self.multistore
-        MultiStore.create_using_pool(ClusterSessions.pool, pool, store_name)
-      end
     end
   end
 end

spec/lib/gitlab/redis/cluster_sessions_spec.rb

-# frozen_string_literal: true
-
-require 'spec_helper'
-
-RSpec.describe Gitlab::Redis::ClusterSessions, feature_category: :scalability do
-  include_examples "redis_new_instance_shared_examples", 'cluster_sessions', Gitlab::Redis::SharedState
-end

spec/lib/gitlab/redis/sessions_spec.rb

@@ -3,7 +3,6 @@
 require 'spec_helper'
 
 RSpec.describe Gitlab::Redis::Sessions, feature_category: :shared do
-  include_examples "multi_store_wrapper_shared_examples"
   include_examples "redis_new_instance_shared_examples", 'sessions', Gitlab::Redis::SharedState
 
   describe '#store' do
@@ -14,9 +13,4 @@
       expect([::Redis::Store, ::Gitlab::Redis::ClusterStore].include?(store.class)).to eq(true)
     end
   end
-
-  it 'migrates from self to ClusterSessions' do
-    expect(described_class.multistore.secondary_pool).to eq(described_class.pool)
-    expect(described_class.multistore.primary_pool).to eq(Gitlab::Redis::ClusterSessions.pool)
-  end
 end