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>