Fix compatibility with new Pulp version.
    
    In latest Pulp version deployed internally, it is no longer possible
    to ask for particular sub-fields using the `fields` argument in the
    query. Pulp now supports only fields which directly match to its
    database fields in `fields` argument.
    
    The fix is simple. Instead of asking for `notes.content_set`, ODCS
    now asks just for `notes`. We will get more data from Pulp than we
    need (all `notes`, not just `notes.content_set`), but this does not
    break anything.