Disable download tests

[stevejalim]

Description

Now that downloads happen on www.firefox.com / mozmeao/springfield and we have the download tests running over there, we should disable the ones running here https://github.com/mozilla/bedrock/actions/workflows/download_tests.yml

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Success Criteria

• Download tests are removed from Bedrock but remain on Springfield

┆Issue is synchronized with this Jira Task
┆Priority: P2
┆Sprint: Backlog

[janbrasna]

Ideally not before #16367

EDIT:
The downloads are still present, esp. from RTAMO — so test removal just hides the visibility, but won't fix any broken links in itself.

(The test covers both /all and /thanks … so both the final download surfaces that have still been kept here for reasons, incl. the site's own /product CTA FWIW. So with more pages redirected, the test surfaces will also be chipped away over time; but not shut down beforehand.)