Delete button not working in Checkout page
Have never done something of that sort but what I assume is, once you move ahead with checkout, a order is created with status IN CHECKOUT or something like that. 

So until a product is there in cart, you can remove that &amp; but once you enter into checkout &amp; order id with other details is created...product can't be removed. Maybe someone from Drupal Commerce maintainer can confirm more on this..