no need for special case default_duration for AAC

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@10126 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b

libavformat/matroskadec.c

@@ -2087,7 +2087,6 @@ matroska_read_header (AVFormatContext    *s,
                 } else {
                     extradata_size = 2;
                 }
-                track->default_duration = 1024*1000 / audiotrack->internal_samplerate;
             }
 
             else if (codec_id == CODEC_ID_TTA) {