x86/apic: remove apic_ops

All supported Xen instances by FreeBSD provide a local APIC
implementation, so there's no need to replace the native local APIC
implementation anymore.

Leave just the ipi_vectored hook in order to be able to override it
with an implementation based on event channels if the underlying local
APIC is not virtualized by hardware. Note the hook cannot use ifuncs,
because at the point where ifuncs are resolved the kernel doesn't yet
know whether it will benefit from using the optimization.

Sponsored by: Citrix Systems R&D
Reviewed by: kib
Differential revision: https://reviews.freebsd.org/D33917