Remove "options PCBGROUP"

With upcoming changes to the inpcb synchronisation it is going to be
broken. Even its current status after the move of PCB synchronization
to the network epoch is very questionable.

This experimental feature was sponsored by Juniper but ended never to
be used in Juniper and doesn't exist in their source tree [sjg@, stevek@,
jtl@]. In the past (AFAIK, pre-epoch times) it was tried out at Netflix
[gallatin@, rrs@] with no positive result and at Yandex [ae@, melifaro@].

I'm up to resurrecting it back if there is any interest from anybody.

Reviewed by: rrs
Differential revision: https://reviews.freebsd.org/D33020