Remove .zenodo.json file (#7148)

I learned only recently that the GitHub-Zenodo integration does not
look at the `.zenodo.json` file if a repository has a `CITATION.cff`
file. Since `CITATION.cff` files are more widely supported (they're
recognized by GitHub, the Zotero browser plugin, and the Zenodo
plugin), we are better off removing `.zenodo.json`. That's one less
file to maintain!

Author: mhucka