Remove questionable Mutex copy code

It is no longer needed, and it had a comment that it needed review...
Now anybody who tries to make a copy again is forced to think about it,
instead of trying and using possibly wrong semantics by accident.

src/support/mutex.cpp

@@ -40,22 +40,6 @@ Mutex::~Mutex()
 }
 
 
-// It makes no sense to copy the mutex,
-// each instance has its own QMutex,
-// therefore nothing to copy!
-// TODO review
-Mutex::Mutex(const Mutex&) : d(new Private)
-{
-}
-
-
-Mutex& Mutex::operator=(const Mutex&)
-{
-	return *this;
-}
-
-
-
 Mutex::Locker::Locker(Mutex* mtx) : mutex_(mtx)
 {
 	mutex_->d->qmutex_.lock();

src/support/mutex.h

@@ -21,6 +21,9 @@ namespace lyx {
 
 class Mutex
 {
+	/// noncopyable
+	Mutex(const Mutex&);
+	Mutex& operator=(const Mutex&);
 public:
 	Mutex();
 	~Mutex();
@@ -46,12 +49,6 @@ public:
 		Mutex* mutex_;
 	};
 
-
-	// pseude-value semantic
-	// needed by GuiPrefs which makes a copy
-	Mutex(const Mutex&);
-	Mutex& operator=(const Mutex&);
-
 private:
 	struct Private;
 	Private* d;