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tcp: remove check for condition that never happens

Description

tcp: remove check for condition that never happens

A tcpcb in TCPS_LISTEN has always socket in SO_ACCEPTCONN. One block
above there is an assertion that proves that this never happens. We
stopped ever clearing SO_ACCEPTCONN back in 779f106aa169.

This reverts commit 982c1675ff8864f51007e0be402ead88429222bb.

Reviewed by: cc, markj
Differential Revision: https://reviews.freebsd.org/D48710