They are unused since CL 114799.

Also remove consts _CTL_KERN and _KERN_OSRELEASE previously used by
getDarwinVersion.

Change-Id: I51b701e8effbe4dd4301b0e6d52e8885469032f4
Reviewed-on: https://go-review.googlesource.com/116955
Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>