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

Removing explicit avio_flush() calls helps us to buffer more data and avoid
flushing the IO context too often which causes reduced IO throughput for
non-streamed file output.

The user can control flushing behaviour at the end of every packet 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 a new 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/img2enc.c

@@ -118,7 +118,6 @@ static int write_packet_pipe(AVFormatContext *s, AVPacket *pkt)
             return ret;
     } else {
         avio_write(s->pb, pkt->data, pkt->size);
-        avio_flush(s->pb);
     }
     img->img_number++;
     return 0;