avformat/flvdec: remove meaningless warning

Ever since the codecpar changes, this has been always printed when
opening a flv file. This is because the codecpar changes made all
streams to be added lazily as read_packet is called.

libavformat/flvdec.c

@@ -1033,7 +1033,6 @@ skip:
         }
         if (i == s->nb_streams) {
             static const enum AVMediaType stream_types[] = {AVMEDIA_TYPE_VIDEO, AVMEDIA_TYPE_AUDIO, AVMEDIA_TYPE_SUBTITLE};
-            av_log(s, AV_LOG_WARNING, "%s stream discovered after head already parsed\n", av_get_media_type_string(stream_types[stream_type]));
             st = create_stream(s, stream_types[stream_type]);
             if (!st)
                 return AVERROR(ENOMEM);