Reimplement checkout item removal. All existing tests now pass

product_in_checkout.rb

@@ -16,6 +16,10 @@ class ProductInCheckout
     @quantity += quantity
   end
 
+  def remove_quantity(quantity)
+    @quantity = [@quantity - quantity, 0].max
+  end
+
   def total
     (@product.price * quantity) - @product.discount.discount_for_quantity(@quantity)
   end