openssh: retire HPN option handling

The HPN patch set was removed from base system SSH in January 2016, in
commit 60c59fad8806. We retained the option parsing (using OpenSSH's
support for deprecated options) to avoid breaking existing installations
upon upgrade, but sufficient time has now passed that we can remove this
special case.

Approved by: des
Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D41291