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»):
and you will see the available action to Delete the task. After you click on it and confirm that you want to delete the task, it will be sent to the trash.
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.