avformat/utils: Move ff_stream_add_bitstream_filter to mux.c

It is muxing-only; in fact, it should be considered part of
the core muxing code.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>

libavformat/internal.h

@@ -771,17 +771,6 @@ enum AVCodecID ff_get_pcm_codec_id(int bps, int flt, int be, int sflags);
  */
 int ff_generate_avci_extradata(AVStream *st);
 
-/**
- * Add a bitstream filter to a stream.
- *
- * @param st output stream to add a filter to
- * @param name the name of the filter to add
- * @param args filter-specific argument string
- * @return  >0 on success;
- *          AVERROR code on failure
- */
-int ff_stream_add_bitstream_filter(AVStream *st, const char *name, const char *args);
-
 /**
  * Copy encoding parameters from source to destination stream
  *