libpmc: remove unused PMC_MDEP_INIT_INTEL_V2

All uses of this macro were removed in e92a1350b50e. Remove
cpu_has_iaf_counters as well.

Reviewed by: ray, emaste
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D30195