xen/intr: remove x86 APIC headers from xen_intr.c

Remove these no longer needed headers. Key for making xen_intr.c
machine-independent as they don't exist on other architectures.

Originally this was part of a much larger commit, but was broken off
for submission to the FreeBSD project.

Reviewed by: royger
Submitted by: Elliott Mitchell <ehem+freebsd@m5p.com>
Original implementation: Julien Grall <julien@xen.org>, 2015-10-20 09:14:56
MFC after: 1 week