btrfs: relocation: simplify unused logic related to LINK_LOWER

btrfs: relocation: simplify unused logic related to LINK_LOWER

btrfs_backref_link_edge() is always called with the LINK_LOWER argument.
We can simplify it and remove the LINK_LOWER and LINK_UPPER macros
completely.

The last call with LINK_UPPER was removed with commit 0097422c0dfe0a
("btrfs: remove clone_backref_node() from relocation").

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: Daniel Vacek <neelx@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>