All architectures enable NEW_PCIB in DEFAULTS (arm being the most recent
to do so in 121be55599 (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