Remove unneeded namespace methods

These methods are not necessary since we changed the `namespace` helper
method in commit 13b3d9ceb.

app/controllers/admin/poll/base_controller.rb

@@ -2,12 +2,4 @@ class Admin::Poll::BaseController < Admin::BaseController
   include FeatureFlags
 
   feature_flag :polls
-
-  helper_method :namespace
-
-  private
-
-    def namespace
-      "admin"
-    end
 end