Put issues group search CTE behind a (default on) flag

We don't think this is needed any more - see
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21521, and
https://gitlab.com/gitlab-org/gitlab-ce/issues/52271 for removing the flag.

app/finders/issuable_finder.rb

@@ -363,6 +363,7 @@ class IssuableFinder
   def use_cte_for_search?
     return false unless search
     return false unless Gitlab::Database.postgresql?
+    return false unless Feature.enabled?(:use_cte_for_group_issues_search, default_enabled: true)
     params[:use_cte_for_search]
   end

spec/controllers/groups_controller_spec.rb

@@ -226,6 +226,11 @@ describe GroupsController do
     end
     context 'searching' do
+      # Remove as part of https://gitlab.com/gitlab-org/gitlab-ce/issues/52271
+      before do
+        stub_feature_flags(use_cte_for_group_issues_search: false)
+      end
+
       it 'works with popularity sort' do
         get :issues, id: group.to_param, search: 'foo', sort: 'popularity'