xen/grant-table: remove unnecessary printing

xen_auto_xlat_grant_frames.vaddr is definitely NULL in this case.
So the address printing is unnecessary.

Signed-off-by: Fuqian Huang <huangfq.daxian@gmail.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>