Remove usesMixer function from the sound streams.

This isn't used anywhere in the codebase. Not even the base SoundStream has it as part of its interface.

Source/Core/AudioCommon/DSoundStream.h

@@ -65,7 +65,6 @@ public:
 	virtual void Stop();
 	virtual void Clear(bool mute);
 	static bool isValid() { return true; }
-	virtual bool usesMixer() const { return true; }
 	virtual void Update();
 
 #else