Remove now-unused WorkerThread ExecutionContextTask.

Source/core/workers/WorkerThread.cpp

 /*
  * Copyright (C) 2008 Apple Inc. All Rights Reserved.
  *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
@@ skipping 226 matching lines @@
         return;
     }
     m_runLoop.terminate();
 }
 
 bool WorkerThread::isCurrentThread() const
 {
     return m_threadID == currentThread();
 }
 
-class ReleaseFastMallocFreeMemoryTask : public ExecutionContextTask {
-    virtual void performTask(ExecutionContext*) OVERRIDE { WTF::releaseFastMallocFreeMemory(); }
-};
-
 } // namespace WebCore