Remove unused get_nested_attached_items util
Remove unused get_nested_attached_items util

indico/indico-plugins#109 removed the need for it, and in any case it
would be something for the plugin if ever needed again.

indico/indico,DirkHoffmann/indico,pferreir/indico,ThiefMaster/indico,DirkHoffmann/indico,indico/indico,ThiefMaster/indico,pferreir/indico,DirkHoffmann/indico,ThiefMaster/indico,ThiefMaster/indico,pferreir/indico,indico/indico,pferreir/indico,indico/indico,DirkHoffmann/indico