FILES: The files provider should not enumerate
    
    Resolves:
    https://pagure.io/SSSD/sssd/issue/3849
    
    For reason I cannot explain now, the files provider always enumerates.
    There is commit a60e6ec which implements this, but it's clearly wrong,
    because then the plain getent passwd output contains duplicates from
    nss_files and nss_sss:
    
    $ getent passwd | sort
    adm:x:3:4:adm:/var/adm:/sbin/nologin
    adm:x:3:4:adm:/var/adm:/sbin/nologin
    bin:x:1:1:bin:/bin:/sbin/nologin
    bin:x:1:1:bin:/bin:/sbin/nologin
    certuser:x:10329:10330::/home/certuser:/bin/bash
    certuser:x:10329:10330::/home/certuser:/bin/bash
    chrony:x:997:994::/var/lib/chrony:/sbin/nologin
    chrony:x:997:994::/var/lib/chrony:/sbin/nologin
    daemon:x:2:2:daemon:/sbin:/sbin/nologin
    daemon:x:2:2:daemon:/sbin:/sbin/nologin
    
    Reviewed-by: Michal Židek <mzidek@redhat.com>