Btrfs: remove deprecated comments

commit d53ba474
(Btrfs: use commit root when loading free space cache) has remove
the deadlock check, and the related comments can be removed as well.

Signed-off-by: Liu Bo <bo.li.liu@oracle.com>
Reviewed-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>