avformat: remove avio_flush() calls from the end of write_header functions

To make it consistent with other muxers.

The user can still control the generic flushing behaviour after write_header
(same way as after packets) using the -flush_packets option, the default
typically means to flush unless a non-streamed file output is used.

Therefore this change should have no adverse effect on streaming, even if it is
assumed that the first packet has a clean buffer, so small seekbacks within the
output buffer work even when the IO context is not seekable.

Signed-off-by: Marton Balint <cus@passwd.hu>

libavformat/avienc.c

@@ -580,8 +580,6 @@ static int avi_write_header(AVFormatContext *s)
     avi->movi_list = ff_start_tag(pb, "LIST");
     ffio_wfourcc(pb, "movi");
 
-    avio_flush(pb);
-
     return 0;
 }