xen/grant-table: Remove unused macro SPP

SPP was used by the grant table v2 code which has been removed in
commit 438b33c7 "xen/grant-table:
remove support for V2 tables".

Signed-off-by: Julien Grall <julien.grall@citrix.com>
Reviewed-by: David Vrabel <david.vrabel@citrix.com>
Cc: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
Cc: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Signed-off-by: David Vrabel <david.vrabel@citrix.com>