ffmpeg: remove dead call to av_parser_change()

It's been a noop for years, and it's been argued that in-band headers
should not be forcedly removed without the user's explicit request.

Also, as the FIXME line stated, this is a job for a bitstream filter
like extract_extradata, remove_extradata, dump_extradata, and
filter_units.

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

fftools/ffmpeg.h

@@ -526,9 +526,6 @@ typedef struct OutputStream {
 
     int keep_pix_fmt;
 
-    AVCodecParserContext *parser;
-    AVCodecContext       *parser_avctx;
-
     /* stats */
     // combined size of all the packets written
     uint64_t data_size;