sctp: Remove special handling for a listen(2) backlog of 0

... when applied to one-to-one-style sockets. sctp_listen() cannot be
used to toggle the listening state of such a socket. See RFC 6458's
description of expected listen(2) semantics for one-to-one- and
one-to-many-style sockets.

Reviewed by: tuexen
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31774