Disable the OpenAL and Pulseaudio audio backends

They are currently broken and cause sound issues which are not present in other
backends:

* OpenAL plays music 2x too fast in Zelda UCode games with HLE
* Pulse backend uses a lot of CPU power and slows down emulation significantly

Both backends are currently being re-implemented in separate branches of
Dolphin, so this should be a temporary removal.

Source/Core/AudioCommon/Src/AudioCommon.cpp

@@ -102,16 +102,12 @@ namespace AudioCommon
 			backends.push_back(BACKEND_DIRECTSOUND);
 		if (XAudio2::isValid())  
 			backends.push_back(BACKEND_XAUDIO2);
-		if (OpenALStream::isValid())
-			backends.push_back(BACKEND_OPENAL);
 		if (AOSound::isValid())   
 			backends.push_back(BACKEND_AOSOUND);
 		if (AlsaSound::isValid()) 
 			backends.push_back(BACKEND_ALSA);
 		if (CoreAudioSound::isValid())       
 			backends.push_back(BACKEND_COREAUDIO);
-		if (PulseAudio::isValid()) 
-			backends.push_back(BACKEND_PULSEAUDIO);
 
 		return backends;
 	}