common/thread: Remove SleepCurrentThread()

This is also unused and superceded by standard functionality. The
standard library provides std::this_thread::sleep_for(), which provides
a much more flexible interface, as different time units can be used with
it.

src/common/thread.h

@@ -83,7 +83,6 @@ private:
     std::size_t generation; // Incremented once each time the barrier is used
 };
 
-void SleepCurrentThread(int ms);
 void SwitchCurrentThread(); // On Linux, this is equal to sleep 1ms
 void SetCurrentThreadName(const char* name);