Remove unneeded namespace methods

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

app/controllers/admin/dashboard/base_controller.rb

@@ -1,9 +1,2 @@
 class Admin::Dashboard::BaseController < Admin::BaseController
-  helper_method :namespace
-
-  private
-
-    def namespace
-      "admin"
-    end
 end