CONFDB: Re-enable the files provider
    
    The files provider was "blacklisted" for a long time, because very old
    (pre-1.0) versions of sssd had the capability to create users and groups
    by calling into the shadow-utils binaries directly which was later
    removed.
    
    Since nobody is (hopefully) running these ancient versions anymore and
    we are about to re-enable the files provider, we can remove this check.
    
    Reviewed-by: Pavel Březina <pbrezina@redhat.com>