sub: remove old MPlayer DVD sub decoder

The DVD sub decoder in Libav 9 was broken/incomplete, so we kept the
MPlayer decoder around. Now it's not needed anymore.

sub/sd_lavc.c

@@ -54,12 +54,7 @@ static bool supports_format(const char *format)
     case AV_CODEC_ID_DVB_SUBTITLE:
     case AV_CODEC_ID_HDMV_PGS_SUBTITLE:
     case AV_CODEC_ID_XSUB:
-    // lavc dvdsubdec doesn't read color/resolution on Libav 9.1 and below,
-    // so fall back to sd_spu in this case. Never use sd_spu with new ffmpeg;
-    // spudec can't handle ffmpeg .idx demuxing (added to lavc in 54.79.100).
-#if LIBAVCODEC_VERSION_INT >= AV_VERSION_INT(54, 40, 0)
     case AV_CODEC_ID_DVD_SUBTITLE:
-#endif
         return true;
     default:
         return false;