sd_ass: remove dead code

With the FFmpeg subtitle decoder used for _all_ non-ASS text subtitle
format, this code is simply unused now.

Ironically, the FFmpeg subtitle decoder does not handle things correctly
in a bunch of cases. Should it turn out they actually matter, they will
have to hack back.

The extend_event one is a candidate, although even though there were
allegedly files which need it, I couldn't get samples from the user who
originally reported such files. As such, extend_event was only confirmed
to handle trailing events with no (endless) duration like with MicroDVD
and LRC, but FFmpeg "fudges" these anyway, so no special handling is
needed.

This code also had logic to handle seeking with muxed srt subtitles,
which made the sub-seek command work. But this has been broken before
this commit already. Currently, seeking with muxed srt subs will clear
all subtitles, as the broken FFmpeg ASS format output by the libavcodec
subtitle converters does not check for duplicates. Since the subtitles
are all cleared, ass_step_sub() can not work properly and sub-seek can
not seek to already seen subtitles.

sub/sd.h

@@ -39,10 +39,6 @@ struct sd {
     struct ass_renderer *ass_renderer;
     pthread_mutex_t *ass_lock;
 
-    // If false, try to remove multiple subtitles.
-    // (Only for decoders which have accept_packets_in_advance set.)
-    bool no_remove_duplicates;
-
     // Set by sub converter
     const char *output_codec;
     char *output_extradata;