xfs: clean up xfs_iunlink_update_inode()

We no longer need to have this function return the previous next
agino value from the on-disk inode as we have it in the in-core
inode now.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Darrick J. Wong <djwong@kernel.org>