Remove the "submit internal subtasks from JSON" feature

[prioux]

An old experimental internal submission system exists in ClusterTask where a running task could create small JSON files that would be picked up by a BourreauWorker, resulting in new tasks being created and submitted.

This is not used at all anymore, all code related to this feature should be removed. See submit_subtasks_from_json() as a starting point.

[Freedisch]

Hello, This sound like a good issue to start with, I'll look into