unix: remove pim sysctl/headers from mksysctl_openbsd.go
unix: remove pim sysctl/headers from mksysctl_openbsd.go

PIM support was removed from OpenBSD in 2016 and as such, the referenced headers
and sysctl names no longer exist.

Change-Id: I7f3e63741b0d4acb8a43e3615b86381f8071e836
Reviewed-on: https://go-review.googlesource.com/c/sys/+/249618
Run-TryBot: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>
TryBot-Result: Gobot Gobot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>
Reviewed-by: Tobias Klauser <0a68dd4915066ec5d3f81f75a828fee53dcc8822@gmail.com>

multiplay/golang-sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys,multiplay/golang-sys,golang/sys