avformat/matroskadec: remove unused variable

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

libavformat/matroskadec.c

@@ -3429,7 +3429,7 @@ static int matroska_read_seek(AVFormatContext *s, int stream_index,
     MatroskaDemuxContext *matroska = s->priv_data;
     MatroskaTrack *tracks = NULL;
     AVStream *st = s->streams[stream_index];
-    int i, index, index_sub, index_min;
+    int i, index, index_min;
 
     /* Parse the CUES now since we need the index data to seek. */
     if (matroska->cues_parsing_deferred > 0) {