avformat/dashenc: removed 'write_manifest' call from 'write_header'

Calling 'write_manifest' from 'write_header' was causing creation of
first MPD with invalid values. Ex: zero @duration param value. Also,
the manifest files (MPD or M3U8s) should be created when at-least
one media frame is ready for consumption.

libavformat/dashenc.c

@@ -1046,9 +1046,6 @@ static int dash_write_header(AVFormatContext *s)
         if ((ret = avformat_write_header(os->ctx, NULL)) < 0)
             return ret;
     }
-    ret = write_manifest(s, 0);
-    if (!ret)
-        av_log(s, AV_LOG_VERBOSE, "Manifest written to: %s\n", s->url);
     return ret;
 }