xen/grant-table: Remove unused macro SPP

xen/grant-table: Remove unused macro SPP

SPP was used by the grant table v2 code which has been removed in
commit 438b33c7145ca8a5131a30c36d8f59bce119a19a "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>
(cherry picked from commit 548f7c94759ac58d4744ef2663e2a66a106e21c5)
Signed-off-by: Elena Ufimtseva <elena.ufimtseva@oracle.com>