Revert of Make run-tests.py warn when it's not testing anything (patchset #1 id:1 of https://codereview.chromium.org/1283513003/ )

Reason for revert:
Some bots that don't run tests (by design) don't return the test results json anymore which makes the infrastructure side fail now.

Original issue's description:
> Make run-tests.py warn when it's not testing anything
>
> I've often been confused by an "all passed" run of the test script
> when it turned out that either all the tests I cared about were skipped
> or, more likely, I mistyped the name(s) of the tests I wanted to run.
> This patch aims to fix that (and gives a useful diagnostic for the
> "all matched tests were skipped" case).
>
> R=machenbach@chromium.org
> NOTRY=true
>
> Committed: https://crrev.com/1b1de2d221a0ed23e529e91bf9fa3dc287acb2ca
> Cr-Commit-Position: refs/heads/master@{#30095}

TBR=jkummerow@chromium.org,adamk@chromium.org
NOPRESUBMIT=true
NOTREECHECKS=true
NOTRY=true

Review URL: https://codereview.chromium.org/1283593005

Cr-Commit-Position: refs/heads/master@{#30097}