runtime: remove unused _CTL_KERN on openbsd

runtime: remove unused _CTL_KERN on openbsd

This was supposed to be removed in CL 539155 already.

Change-Id: I22be1a72d5d8eb02514045609429e706c344d5fc
Reviewed-on: https://go-review.googlesource.com/c/go/+/542915
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