btrfs: remove unused btrfs_ordered_extent::outstanding_isize

The whole isize code was deleted in 5.6 3f1c64ce ("btrfs: delete the
ordered isize update code"), except the struct member.  This was found
by tool https://github.com/jirislaby/clang-struct

 .

Reviewed-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>