Remove orphaned tasks from cylc workflows

When variables are removed from that variable list partway through processing, we can sometimes end up with "orphaned tasks" that no longer have any variables to process. These tasks will fail on all cycles until the task is removed from the `rose-suite.conf` or the workflow is re started from scratch so that the task is never introduced.

We need some form of utility to allow CDDS to cope when this happens. Some options could be:
- Remove the task from the `rose-suite.conf` directly and reload the workflow
- Set all tasks that fail with "nothing to do" as succeeded in a way that cylc will continue processing without giving errors in the finaliser tasks.