codecov.io integration broke with GitHub Actions migration

[eloquence]

We're currently no longer getting codecov.io reports:
https://app.codecov.io/gh/freedomofpress/securedrop

This appears to be because our manual Codecov upload code is CircleCI-specific (e.g., it only runs if CIRCLE_BRANCH is set).

We should decide either to retire the integration once and for all (the fact that we've not even noticed it is broken suggests we may not be getting a lot of value out of it at this time), or to fix it to work with GHA.

[legoktm]

IIRC it broke even earlier, which is why we didn't try to migrate it. But I think we can just get rid of it now; we still have local coverage reports.