The pmap function pmap_clear_reference() is no longer used.  Remove it.

pmap_clear_reference() has had exactly one caller in the kernel for
several years, more precisely, since FreeBSD 8.  Now, that call no
longer exists.

Approved by:	re (kib)
Sponsored by:	EMC / Isilon Storage Division
sys/i386/xen/mp_machdep.c:
sys/i386/xen/mptable.c:
	Set PCPU apic_id and acpi_id fields in a fasion compatible with
	both UP and SMP configurations.

Suggested by:	jhb
Submitted by:	Roger Pau Monné
Sponsored by:	Citrix Systems R&D
Reviewed by:	gibbs
Approved by:	re (blanket Xen)
MFC after:	2 weeks