CI: Remove nightly codecov job

[clonker]

No description provided.

[cameel]

We should probably get rid of codecov.yml as well. There was this doubt whether fuzzing depends on codecov, but I somehow doubt it. I'd say let's remove it and we can always restore if things break.

Also docker images install the codecov package, which is probably also unnecessary. Though this would require rebuilding the images, which makes it a hassle, so that may be better done later, together with evmone update.