avformat: remove FF_API_INIT_PACKET from AVStream.attached_pic

This field needs to be replaced altogether, not just its type changed.
This will be done in a separate change.

Signed-off-by: James Almer <jamrial@gmail.com>

libavformat/avformat.h

@@ -954,11 +954,7 @@ typedef struct AVStream {
      * decoding: set by libavformat, must not be modified by the caller.
      * encoding: unused
      */
-#if FF_API_INIT_PACKET
     AVPacket attached_pic;
-#else
-    AVPacket *attached_pic;
-#endif
 
     /**
      * An array of side data that applies to the whole stream (i.e. the