Remove Codecov as a CI step on master/production branches

[kokrui]

As mentioned in https://github.com/nusmodifications/nusmods/issues/3688, I think we should remove Codecov as a CI step on the master and production branches.

I think we should keep it on pull-requests so we can still see how coverage changes each time a PR is made / while reviewing PRs, but once a commit has landed in master / prod it's no longer useful information.