Clean up instances that are unused since CL 6450058.

Change-Id: I0e9ae28cfa83fcc8abda8f5eca9c7dfc2c1c4ad1
Reviewed-on: https://go-review.googlesource.com/c/go/+/477396


Reviewed-by: Michael Pratt <mpratt@google.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Run-TryBot: Ian Lance Taylor <iant@google.com>
Auto-Submit: Ian Lance Taylor <iant@golang.org>
Auto-Submit: Ian Lance Taylor <iant@google.com>
Reviewed-by: Ian Lance Taylor <iant@google.com>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Bypass: Ian Lance Taylor <iant@golang.org>