Deletion through XOG is only supported for some objects ; not sure whether task/dependencies are supported though.
If it were supported what you would have to do is provide the entire XML for the task (inculcating any dependencies that you did not want to delete) but without the dependency you wish to delete, and then add the complete="true" flag to the XML.
(but note I really don't know if that works in this case - if I were to GUESS I would say that you can't do it)
Search for "delete xog" on the community for lots of discussions, but nothing obviously related to your specific issue.