tcp: remove tcptw, the compressed timewait state structure

The memory savings the tcptw brought back in 2003 (see 340c35de6a2) no
longer justify the complexity required to maintain it. For longer
explanation please check out the email [1].

Surpisingly through almost 20 years the TCP stack functionality of
handling the TIME_WAIT state with a normal tcpcb did not bitrot. The
existing tcp_input() properly handles a tcpcb in TCPS_TIME_WAIT state,
which is confirmed by the packetdrill tcp-testsuite [2].

This change just removes tcptw and leaves INP_TIMEWAIT. The flag will
be removed in a separate commit. This makes it easier to review and
possibly debug the changes.

[1] https://lists.freebsd.org/archives/freebsd-net/2022-January/001206.html
[2] https://github.com/freebsd-net/tcp-testsuite

Differential revision: https://reviews.freebsd.org/D36398