tcp: remove a condition in tcp_usr_detach() that never happens

The comment from Robert Watson doubts that this condition ever happens.
Our analysis confirm that. Also, we found that if you manage to create
such a connection with help of some other bug, then after the "second
case" code is executed, the kernel will panic in other part of the stack.

Reviewed by: rrs, tuexen
Differential revision: https://reviews.freebsd.org/D35714