Use User#permitted_params in Users::RolesController#user_params

To further simplify the method, c.f. this commit [1] where
User#permitted_params was introduced.

[1]: 09ff60d

app/controllers/users/roles_controller.rb

@@ -29,8 +29,7 @@ def authorize_user
   end
 
   def user_params
-    permitted_user_params = current_user.role_class.permitted_user_params
-    params.require(:user).permit(*permitted_user_params.intersection([:role]))
+    params.require(:user).permit(*current_user.permitted_params.intersection([:role]))
   end
 
   def redirect_to_account_page_if_acting_on_own_user