Remove IsolateSpawnState::Cleanup as it is not used anywhere.

runtime/vm/isolate.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "vm/isolate.h"
 
 #include "include/dart_api.h"
 #include "include/dart_native_api.h"
 #include "platform/assert.h"
 #include "platform/json.h"
@@ skipping 2594 matching lines @@
   return DeserializeObject(thread, serialized_args_, serialized_args_len_);
 }
 
 
 RawInstance* IsolateSpawnState::BuildMessage(Thread* thread) {
   return DeserializeObject(thread,
                            serialized_message_, serialized_message_len_);
 }
 
 
-void IsolateSpawnState::Cleanup() {
-  SwitchIsolateScope switch_scope(I);
-  Dart::ShutdownIsolate();
-}
-
 }  // namespace dart