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/AOSoundStream.h

@@ -41,10 +41,6 @@ public:
 		return true;
 	}
 
-	virtual bool usesMixer() const {
-		return true;
-	}
-
 	virtual void Update() override;
 
 #else

Source/Core/AudioCommon/AlsaSoundStream.h

@@ -26,9 +26,6 @@ public:
 	static bool isValid() {
 		return true;
 	}
-	virtual bool usesMixer() const {
-		return true;
-	}
 
 	virtual void Update() override;
 

Source/Core/AudioCommon/CoreAudioSoundStream.h

@@ -25,9 +25,6 @@ public:
 	static bool isValid() {
 		return true;
 	}
-	virtual bool usesMixer() const {
-		return true;
-	}
 
 	virtual void Update();
 

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

Source/Core/AudioCommon/NullSoundStream.h

@@ -27,6 +27,5 @@ public:
 	virtual void Stop() override;
 	virtual void Clear(bool mute) override;
 	static bool isValid() { return true; }
-	virtual bool usesMixer() const { return true; }
 	virtual void Update() override;
 };

Source/Core/AudioCommon/OpenALStream.h

@@ -57,7 +57,6 @@ public:
 	virtual void Stop() override;
 	virtual void Clear(bool mute) override;
 	static bool isValid() { return true; }
-	virtual bool usesMixer() const { return true; }
 	virtual void Update() override;
 
 private:

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;

Source/Core/AudioCommon/PulseAudioStream.h

@@ -25,8 +25,6 @@ public:
 
 	static bool isValid() {return true;}
 
-	virtual bool usesMixer() const {return true;}
-
 	virtual void Update() override;
 
 	void StateCallback(pa_context *c);

Source/Core/AudioCommon/XAudio2Stream.h

@@ -60,7 +60,6 @@ public:
 	virtual void Update();
 	virtual void Clear(bool mute);
 	virtual void SetVolume(int volume);
-	virtual bool usesMixer() const { return true; }
 
 	static bool isValid() { return InitLibrary(); }
 

Source/Core/AudioCommon/XAudio2_7Stream.cpp

@@ -260,8 +260,6 @@ void XAudio2_7::Stop()
 	}
 }
 
-bool XAudio2_7::usesMixer() const { return true; }
-
 #else
 
 struct StreamingVoiceContext2_7 {};
@@ -283,7 +281,6 @@ void XAudio2_7::Stop() {}
 void XAudio2_7::Update() {}
 void XAudio2_7::Clear(bool mute) {}
 void XAudio2_7::SetVolume(int volume) {}
-bool XAudio2_7::usesMixer() const { return false; }
 bool XAudio2_7::InitLibrary() { return false; }
 
 #endif

Source/Core/AudioCommon/XAudio2_7Stream.h

@@ -66,7 +66,6 @@ public:
 	virtual void Update();
 	virtual void Clear(bool mute);
 	virtual void SetVolume(int volume);
-	virtual bool usesMixer() const;
 
 	static bool isValid() { return InitLibrary(); }