lavf/internal: remove a prototype for non-existent function

ff_stream_side_data_copy() has been gone since
5432d2aaca

libavformat/internal.h

@@ -660,15 +660,6 @@ int ff_is_intra_only(enum AVCodecID id);
  */
 enum AVCodecID ff_get_pcm_codec_id(int bps, int flt, int be, int sflags);
 
-/**
- * Copy side data from source to destination stream
- *
- * @param dst pointer to destination AVStream
- * @param src pointer to source AVStream
- * @return >=0 on success, AVERROR code on error
- */
-int ff_stream_side_data_copy(AVStream *dst, const AVStream *src);
-
 /**
  * Create a new stream and copy to it all parameters from a source stream, with
  * the exception of the index field, which is set when the new stream is