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/OpenSLESStream.h

@@ -20,7 +20,6 @@ public:
 	virtual bool Start();
 	virtual void Stop();
 	static bool isValid() { return true; }
-	virtual bool usesMixer() const { return true; }
 
 private:
 	std::thread thread;