You can delete a task from editing a post or from the editorial calendar.
To delete a task within the edit post page, just hover the task you want to delete with your mouse (for instance, «Translate post»):
so that available actions become visible:
The only possible action for a task—besides completing it—is removing the task. Click on the Trash icon to delete the task.
From the editorial calendar, you can also delete an editorial task. To do so, simply select the task you want to delete from the calendar and drag it to the trash.
And that’s it, that task will have been deleted.