Disable Codecov status messages

[wilzbach]

Due to many random coverage changes, the current CodeCov is suboptimal
at Druntime as it triggers many red crosses for perfectly healthy PRs.
As there's no way to configure CodeCov nor GitHub to change the
behavior to not showing an overall failed status for coverage failures,
the simple solution is to turn the CodeCov status notifications off.
However, in the future the Dlang-Bot might display the CodeCov summary
as part of his comment.
Furthermore, the CodeCov commits will continue to get logged at [1], a
summary coverage for each file on e.g. master can always be found at [2] and
the CodeCov browser extension [3] will be working as well.

[1] https://codecov.io/gh/dlang/druntime
[2] https://codecov.io/gh/dlang/druntime/list/master
[3] https://github.com/codecov/browser-extension

[dlang-bot]

Thanks for your pull request, @wilzbach!

Bugzilla references

Your PR doesn't reference any Bugzilla issue.

If your PR contains non-trivial changes, please reference a Bugzilla issue or create a manual changelog.

[wilzbach]

(rebased to rekick DAutoTest)

@CyberShadow FYI: I added a simple dumping codecov_hook to the bot and added it here to the YAML, s.t. we already can see what kind of data the people from CodeCov send us (doc seem to be very sparse).

[CyberShadow]

Alright :)