As far as I know, it's not possible to hide a queue from the assignments list. If we had access to the set ownership and reassign workflows, you would be able to remove them.
Essentially, the assignments dropdown is controlled by the queues. From what I understand from the documentation, the actual assignments of the ticket are the groups and users that are in the queues. You might be able to use the send to workflow action on a rule and do the assignment to a Desktop Applications (**) group using that. I don't think this is a good idea though and it's probably not supported but you're free to try doing so.