Simplify implementation of Mutex.

src/platform-nullos.cc

Index: src/platform-nullos.cc
diff --git a/src/platform-nullos.cc b/src/platform-nullos.cc
index dd5a3ddb32fcfaf52037e6300ea5498048777467..eadb300c290a1223badd96670489790413fb1a32 100644
--- a/src/platform-nullos.cc
+++ b/src/platform-nullos.cc
@@ -477,37 +477,6 @@ void Thread::YieldCPU() {
 }
 
 
-class NullMutex : public Mutex {
- public:
-  NullMutex() : data_(NULL) {
-    UNIMPLEMENTED();
-  }
-
-  virtual ~NullMutex() {
-    UNIMPLEMENTED();
-  }
-
-  virtual int Lock() {
-    UNIMPLEMENTED();
-    return 0;
-  }
-
-  virtual int Unlock() {
-    UNIMPLEMENTED();
-    return 0;
-  }
-
- private:
-  void* data_;
-};
-
-
-Mutex* OS::CreateMutex() {
-  UNIMPLEMENTED();
-  return new NullMutex();
-}
-
-
 class NullSemaphore : public Semaphore {
  public:
   explicit NullSemaphore(int count) : data_(NULL) {