build: remove unused, outdated cloudbuild config

The config predates kokoro CI.
The GitHub-based test workflow is not accessible
by external, unapproved contributors.

Remove the docs/testing.md that mostly talks about
these outdated workflows.

Change-Id: Ie7502067d53494f9b28bc807313b35885e066cb3
Reviewed-on: https://go-review.googlesource.com/c/vscode-go/+/506655
Reviewed-by: Suzy Mueller <suzmue@golang.org>
Run-TryBot: Hyang-Ah Hana Kim <hyangah@gmail.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>

Author: hyangah