build: remove unused, outdated cloudbuild config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these outdated workflows.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 <[email protected]>
Run-TryBot: Hyang-Ah Hana Kim <[email protected]>
TryBot-Result: kokoro <[email protected]>

Author: hyangah