backend: use createrepo_mod tool for generating module repodata
    
    Instead of knowing the specifics of module repositories format,
    let's outsource it to a `createrepo_mod` tool from `modulemd-tools`
    package.
    
    The `backend/copr_backend/createrepo.py` now seems to be useless
    for module repo generation because it is done by `copr-repo` script,
    so I am dropping the code.