Remove dsound audio backend.

There isn't any reason to use dsound over xaudio.

Source/Core/DolphinWX/ConfigMain.cpp

@@ -1004,8 +1004,7 @@ bool CConfigMain::SupportsVolumeChanges(std::string backend)
 	//FIXME: this one should ask the backend whether it supports it.
 	//       but getting the backend from string etc. is probably
 	//       too much just to enable/disable a stupid slider...
-	return (backend == BACKEND_DIRECTSOUND ||
-			backend == BACKEND_COREAUDIO ||
+	return (backend == BACKEND_COREAUDIO ||
 			backend == BACKEND_OPENAL ||
 			backend == BACKEND_XAUDIO2);
 }