Rip out all code that implements the subtasking mechanism

[prioux]

We have code that allows a running cluster job to prepare a JSON description (unrelated to Boutiques) for new cluster jobs to be created within CBRAIN. CBRAIN will poll for these JSON files and created new tasks. It was needed only in a experimental project, we don't need this anymore.

Code to look for: submit_subtasks_from_json in cluster_task.rb (and check all related commits elsewhere).