xen/x86: there's no highmem anymore in PV mode

Considerations for it are a leftover from when 32-bit was still
supported.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>
Link: https://lore.kernel.org/r/ba6e0779-18f4-ae64-b216-73205b4eec3c@suse.com


Signed-off-by: Boris Ostrovsky <boris.ostrovsky@oracle.com>