Move Cubeb AudioStream to FrontendCommon

This way it doesn't need to be built for libretro.

src/common/audio_stream.h

@@ -47,8 +47,6 @@ public:
 
   static std::unique_ptr<AudioStream> CreateNullAudioStream();
 
-  static std::unique_ptr<AudioStream> CreateCubebAudioStream();
-
   // Latency computation - returns values in seconds
   static float GetMaxLatency(u32 sample_rate, u32 buffer_size);