tcp: remove unused code in tcp_usr_attach

pr_attach is only called on a socket (so) with so->so_listen != NULL
via sonewconn. However, sonewconn is not called from the TCP code.
The listening sockets are handled in tcp_syncache.c without using
sonewconn. Therefore, the code removed is never executed.
No functional change intended.

Reviewed by: rrs, peter.lei_ieee.org
MFC after: 1 week
Sponsored by: Netflix, Inc.
Differential Revision: https://reviews.freebsd.org/D45412