fixing recursive deletion of domains

Summary:
recursive deletion: need to find and delete objects with objectclass ldapsubentry as well (#5100).
This is for example needed when deleting a domain, for the role kolab-admin which has class ldapsubentry

Reviewers: vanmeeuwen, #web_administration_panel_developers

Reviewed By: vanmeeuwen, #web_administration_panel_developers

Subscribers: machniak, vanmeeuwen

Differential Revision: https://git.kolab.org/D23