tcp: remove an invalid KASSERT

Since TCP endpoints in the TIME-WAIT state are no longer special,
allow them to be switched over to the default stack.

Reported by: syzbot+902d31bb0e68a1e8c088@syzkaller.appspotmail.com
Reviewed by: cc
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D51090