How to remove the submit button on the form included by drupal_get_form?
If you look at the function drupal_render it will ignore all elements marked as #printed so just add that to the submit button's array.

You'll need to inspect the $delivery_information_form array to see exactly where your button is at, and when you find it you can do something like this:

  $delivery_information_form = drupal_get_form('sbsl_order_delivery_information_1_form_' . $supplier_id);
  $delivery_information_form['path']['to']['button']['#printed'] = TRUE;


before adding it to the $element array.