In Slack, find the task that you want to remove the due date by typing /todo list in the channel
Click on the blue link which is the Task ID which will take you to the To-do Dashboard
Alternatively you can login to the To-do Dashboard by clicking this link
This will take you to the Task Details Page in the Dashboard.
Click on the due date, and scroll down and find the Remove button
Click on this button to remove the due date