confdb: Do not start implicit_files with proxy domain
    
    id_provider = proxy + proxy_lib_name = files is equivalent
    to id_provider = files. But requests to user hit implicit_files
    domain instead of proxy domain and therefore it broke usage
    of proxy domain with auth_provider = krb5.
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3590
    
    Reviewed-by: Jakub Hrozek <jhrozek@redhat.com>