tcp: remove support for TCPPCAP

This feature could be used to store the last sent and received TCP
packets for a TCP endpoint. There was no utility to get these packets
from a live system or core.
This functionality is now provided by TCP Black Box Logging, which also
stores additional events. There are tools to get these traces from a
live system or a core.
Therefore remove TCPPCAP to avoid maintaining it, when it is not
used anymore.

Reviewed by: rrs, rscheff, Peter Lei, glebiu
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D49589