Retire non-NEW_PCIB code and remove config option

All architectures enable NEW_PCIB in DEFAULTS (arm being the most recent
to do so in 121be555997b (arm: Set NEW_PCIB in DEFAULTS rather than a
subset of kernel configs")), so it's time we removed the legacy code
that no longer sees much testing and has a significant maintenance
burden.

Reviewed by: jhb, andrew, emaste
Differential Revision: https://reviews.freebsd.org/D32954