ssh: remove ssh-hpn leftovers

This was introduced in 8998619212f3a, and left behind when the hpn-ssh
patches were removed in 60c59fad8806. Although Being able to log
SO_RCVBUF in debug mode might have some small value on its own, it's
not worth carrying an extra diff against upstream.

Reviewed by: kevans
MFC after: 3 days
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D28610