Deleting a task doesn't clean up references to the deleted task key

[eatyourgreens]

Expected behavior

Deleting a task should delete all references to its task key, within that workflow.

Current behavior

task.next isn't cleaned up after deleting a task, leading to workflow tasks where task.next points to a deleted task key.

Steps to replicate

1. Create a workflow such that T1 -> T2 -> Submit classification.
2. Delete task T2.
3. Examine task T1 to see that T1.next is still set to 'T2'.

Additional information

• this bug is the cause of Task order not reflecting Lab choice + freezing workflow front-end-monorepo#5807.

[eatyourgreens]

Branching single answer questions that link to the deleted task are not cleaned up either.

[goplayoutside3]

Linking the culprit function too:

Panoptes-Front-End/app/pages/lab/workflow.jsx

Lines 797 to 814 in 44da424

	handleTaskDelete(taskKey, e) {
	const shortcut = this.props.workflow.tasks[taskKey].unlinkedTask;
	if (e.shiftKey || confirm('Really delete this task?')) {
	if (this.canUseTask(this.props.project, "transcription-task")) {
	this.deleteStepAndTask(taskKey);
	} else {
	const changes = {};
	if (shortcut) {
	changes[`tasks.${shortcut}`] = undefined;
	}
	changes[`tasks.${taskKey}`] = undefined;
	this.props.workflow.update(changes);
	}
	return this.updateFirstTask();
	}
	}
	}