Merge pull request rails#55479 from Shopify/drop-unused-dispatcher_class-override

Remove unused `dispatcher_class` override from `MapperTest::FakeSet`

Author: rafaelfranca

actionpack/test/dispatch/mapper_test.rb

@@ -14,10 +14,6 @@ def request_class
           ActionDispatch::Request
         end
 
-        def dispatcher_class
-          RouteSet::Dispatcher
-        end
-
         def defaults
           routes.map(&:defaults)
         end