!tes #3544 Removed first parameter from TestKit.shutdown

This PR removes the first parameter (actorSystem) from `TestKit.shutdown`. Instead `shutdown` now uses the `ActorSystem` associated with the `TestKit` instance. Every occurrence of `TestKit.shutdown(actorSystem)` in the akka codebase has been replaced by either `shutdown()` or `TestKit.shutdownActorSystem(actorSystem)`.