arm/xen: remove the unused macro GRANT_TABLE_PHYSADDR

Fix the following sparse warning:

arch/arm64/xen/../../arm/xen/enlighten.c:244: warning: macro
"GRANT_TABLE_PHYSADDR" is not used [-Wunused-macros]

It is an isolated macro, and should be removed when its last user
was deleted in the following commit 3cf4095d7446 ("arm/xen: Use
xen_xlate_map_ballooned_pages to setup grant table")

Signed-off-by: Xiaofei Tan <[e-mail]>
Reviewed-by: Stefano Stabellini <[e-mail]>
Signed-off-by: Stefano Stabellini <[e-mail]>

arch/arm/xen/enlighten.c

···
  * see Documentation/devicetree/bindings/arm/xen.txt for the
  * documentation of the Xen Device Tree format.
  */
-#define GRANT_TABLE_PHYSADDR 0
 void __init xen_early_init(void)
 {
 	of_scan_flat_dt(fdt_find_hyper_node, NULL);