trustdomain-del: fix the way how subdomain is searched
    
    With FreeIPA 4.4 we moved child domains behind the 'trustdomain' topic.
    Update 'ipa trustdomain-del' command to properly calculate DN to the
    actual child domain and handle the case when it is missing correctly.
    
    Fixes https://fedorahosted.org/freeipa/ticket/6445
    
    Reviewed-By: Martin Babinsky <mbabinsk@redhat.com>