Btrfs: get rid of BTRFS_INODE_HAS_ORPHAN_ITEM

Now that we don't add orphan items for truncate, there can't be races on
adding or deleting an orphan item, so this bit is unnecessary.

Reviewed-by: Nikolay Borisov <nborisov@suse.com>
Signed-off-by: Omar Sandoval <osandov@fb.com>
Signed-off-by: David Sterba <dsterba@suse.com>