runtime: remove unused cgoCheckUsingType function

runtime: remove unused cgoCheckUsingType function

The only calls to it were removed in CL 616255.

Change-Id: I6c6b01e2e98d54300b6323fd74ccc45fa1d433dd
Reviewed-on: https://go-review.googlesource.com/c/go/+/714820
Reviewed-by: Tobias Klauser <tobias.klauser@gmail.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Michael Knyszek <mknyszek@google.com>
Reviewed-by: David Chase <drchase@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>