avformat: remove more unneeded avio_flush() calls

avformat: remove more unneeded avio_flush() calls

These instances are simply redundant or present because avio_flush() used to be
required before doing a seekback. That is no longer the case, aviobuf code does
the flush automatically on seek.

This only affects code which is either disabled for streaming IO contexts or
does no seekbacks after the flush, so this change should have no adverse effect
on streaming.

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