Sequence tasks can't be deleted, but you can skip them from the Tasks screen if you can't complete them. When you skip a task, it's like deleting a manually-created task: the task will disappear from your task list completely.
Tasks that are skipped will be counted in your sequence metrics, so you can see if your sequence needs some fine-tuning.
Tip: If you're an admin, you can skip multiple tasks at once using bulk actions.
In the Tasks screen, click the options icon on the task you want to edit.