Purpose of codecov is to:
1. show a web UI of lines that need coverage
2. sanity-check PRs
3. show a pretty badge on README
codecov (and/or gcov) is not reliable enough to allow it to cause
a "red" status in the `master` branch CI history.
It fails PRs which otherwise would succeed (e.g. #8971), and the Codecov
status page doesn't load, so we can't see what it's complaining about.
Reverts 5c7237640f
ref #8910
