Remove subscription_plan_cache_key feature flag

ee/app/services/fetch_subscription_plans_service.rb

@@ -39,10 +39,6 @@ class FetchSubscriptionPlansService
   end
 
   def cache_key
-    if Feature.enabled?(:subscription_plan_cache_key)
-      "subscription-plan-#{@plan}"
-    else
-      "subscription-plans-#{@plan}"
-    end
+    "subscription-plan-#{@plan}"
   end
 end

ee/config/feature_flags/development/subscription_plan_cache_key.yml

----
-name: subscription_plan_cache_key
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/43973
-rollout_issue_url:
-type: development
-group: group::fulfillment
-default_enabled: false