Cleanup the form action URLs generated on the product listing pages

Cleanup the form action URLs generated on the product listing pages. More specifically stop the generation of duplicate "products_id" parameters.

When a customer adds a product to the shopping cart from a product listing page (with the store set to show the shopping cart) the "continue shopping" (back) button includes the "products_id" parameter. When the customer clicks the "continue shopping" button the customer is sent to the product listing page with "products_id" in the URL. This currently causes generated forms on the product listing page to include the "products_id" parameter twice in the form's action URL (once with the last added product id and a second time with the id of the product to be added).