Removes namespace_limits_admin_dashboard FF

With the completion of the admin UI for namespace limit controls, we can
remove the feature flag that was preventing it being available

Changelog: added
EE: true

config/feature_flags/development/namespace_limits_admin_dashboard.yml

----
-name: namespace_limits_admin_dashboard
-introduced_by_url: https://gitlab.com/gitlab-org/gitlab/-/merge_requests/115704
-rollout_issue_url: https://gitlab.com/gitlab-org/gitlab/-/issues/399220
-milestone: '15.11'
-type: development
-group: group::utilization
-default_enabled: false

ee/app/controllers/admin/namespace_limits_controller.rb

@@ -5,7 +5,6 @@ class NamespaceLimitsController < Admin::ApplicationController
     feature_category :consumables_cost_management
     urgency :low
 
-    before_action :check_dashboard_enabled
     before_action :check_gitlab_com
 
     def index; end
@@ -15,9 +14,5 @@ def index; end
     def check_gitlab_com
       not_found unless ::Gitlab::CurrentSettings.should_check_namespace_plan?
     end
-
-    def check_dashboard_enabled
-      not_found unless ::Feature.enabled?(:namespace_limits_admin_dashboard, current_user)
-    end
   end
 end

ee/spec/requests/admin/namespace_limits_controller_spec.rb

@@ -38,14 +38,6 @@
       context 'when not on .com' do
         it_behaves_like 'not found'
       end
-
-      context 'when :namespace_limits_admin_dashboard is disabled' do
-        before do
-          stub_feature_flags(namespace_limits_admin_dashboard: false)
-        end
-
-        it_behaves_like 'not found'
-      end
     end
 
     context 'with non-admin user' do