output/Filtered: remove BeginDestroy()

All it does is "sometimes" close the mixer, but that is also done in
the destructor.

src/output/Control.cxx

@@ -371,8 +371,6 @@ AudioOutputControl::LockCloseWait() noexcept
 void
 AudioOutputControl::BeginDestroy() noexcept
 {
-	output->BeginDestroy();
-
 	if (thread.IsDefined()) {
 		const std::lock_guard<Mutex> protect(mutex);
 		CommandAsync(Command::KILL);

src/output/Filtered.hxx

@@ -141,8 +141,6 @@ public:
 		   MixerListener &mixer_listener,
 		   const ConfigBlock &block);
 
-	void BeginDestroy() noexcept;
-
 	const char *GetName() const {
 		return name;
 	}

src/output/Finish.cxx

@@ -28,10 +28,3 @@ FilteredAudioOutput::~FilteredAudioOutput()
 	if (mixer != nullptr)
 		mixer_free(mixer);
 }
-
-void
-FilteredAudioOutput::BeginDestroy() noexcept
-{
-	if (mixer != nullptr)
-		mixer_auto_close(mixer);
-}