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>