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 3cf4095d

 ("arm/xen: Use
xen_xlate_map_ballooned_pages to setup grant table")

Signed-off-by: Xiaofei Tan <tanxiaofei@huawei.com>
Reviewed-by: Stefano Stabellini <sstabellini@kernel.org>
Signed-off-by: Stefano Stabellini <sstabellini@kernel.org>