runtime: remove unused getOSRev on openbsd

runtime: remove unused getOSRev on openbsd

It's unused since CL 538458.

Change-Id: Ic8d30b0fb54f3f1d723626c5db56fbf4cf181dea
Reviewed-on: https://go-review.googlesource.com/c/go/+/539155
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
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Reviewed-by: Joel Sing <joel@sing.id.au>
Auto-Submit: Tobias Klauser <tobias.klauser@gmail.com>
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Run-TryBot: Tobias Klauser <tobias.klauser@gmail.com>
Reviewed-by: Bryan Mills <bcmills@google.com>