xen, arm64: drop dummy lookup_address()

This is unused, and conflicts with the definition that we'll add for XPFO.

Signed-off-by: Tycho Andersen <tycho@docker.com>
Reviewed-by: Julien Grall <julien.grall@arm.com>
CC: Boris Ostrovsky <boris.ostrovsky@oracle.com>
CC: Juergen Gross <jgross@suse.com>
CC: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>