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

runtime/vm/isolate.h

 // 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.
 
 #ifndef VM_ISOLATE_H_
 #define VM_ISOLATE_H_
 
 #include "include/dart_api.h"
 #include "platform/assert.h"
 #include "vm/atomic.h"
@@ skipping 991 matching lines @@
   const char* class_name() const { return class_name_; }
   const char* function_name() const { return function_name_; }
   bool is_spawn_uri() const { return library_url_ == NULL; }
   bool paused() const { return paused_; }
   bool errors_are_fatal() const { return errors_are_fatal_; }
   Isolate::Flags* isolate_flags() { return &isolate_flags_; }
 
   RawObject* ResolveFunction();
   RawInstance* BuildArgs(Thread* thread);
   RawInstance* BuildMessage(Thread* thread);
-  void Cleanup();
 
  private:
   Isolate* isolate_;
   Dart_Port parent_port_;
   Dart_Port on_exit_port_;
   Dart_Port on_error_port_;
   const char* script_url_;
   const char* package_root_;
   const char** package_map_;
   const char* library_url_;
   const char* class_name_;
   const char* function_name_;
   uint8_t* serialized_args_;
   intptr_t serialized_args_len_;
   uint8_t* serialized_message_;
   intptr_t serialized_message_len_;
   Isolate::Flags isolate_flags_;
   bool paused_;
   bool errors_are_fatal_;
 };
 
 }  // namespace dart
 
 #endif  // VM_ISOLATE_H_