Remove some unneeded #includes and forward declarations

src/AudioIOListener.h

@@ -15,6 +15,10 @@
 
 #include "Audacity.h"
 
+class WaveTrack;
+using WaveTrackArray =
+   std::vector < std::shared_ptr < WaveTrack > >;
+
 class AUDACITY_DLL_API AudioIOListener /* not final */ {
 public:
    AudioIOListener() {}