x86/xen: remove PVHv1 code
PVHv1 was officially removed from Xen in 4.9, so just axe the related
code from FreeBSD.
Note FreeBSD supports PVHv2, which is the replacement for PVHv1.
Sponsored by: Citrix Systems R&D
Reviewed by: kib, Elliott Mitchell
Differential Revision: https://reviews.freebsd.org/D30228