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>

fs/btrfs/ordered-data.h

@@ -96,13 +96,6 @@ struct btrfs_ordered_extent {
 	/* number of bytes that still need writing */
 	u64 bytes_left;
 
-	/*
-	 * the end of the ordered extent which is behind it but
-	 * didn't update disk_i_size. Please see the comment of
-	 * btrfs_ordered_update_i_size();
-	 */
-	u64 outstanding_isize;
-
 	/*
 	 * If we get truncated we need to adjust the file extent we enter for
 	 * this ordered extent so that we do not expose stale data.