Не могу удалить элемент объекта, созданный конструктором

class Product { 
  constructor(name, size) { 
    this.name = name; 
    this.size = size; 
  } 
 
  getSize() { 
    return this.size; 
  } 
} 
 
class Burger extends Product { 
  constructor(name, size, stuffing) { 
    super(name, size); 
    this.stuffing = stuffing; 
  } 
 
  getStuffing() { 
    return this.stuffing; 
  } 
} 
 
class Menu { 
  constructor(burger, salad, drink) { 
    this.burger = burger; 
  } 
 
  orderBill() { 
    let burgerSizePrice = 
      this.burger.getSize() === "xl" 
        ? 250 
        : this.burger.getSize() === "normal" 
        ? 200 
        : 150; 
 
 
    let burgerStuffingPrice = 
      this.burger.getStuffing() === "xl" 
        ? 100 
        : this.burger.getStuffing() === "normal" 
        ? 80 
        : 50; 
 
 
    let result =  burgerSizePrice + burgerStuffingPrice; 
    return result; 
  } 
 
  deleteProduct(product){ 
    delete this[product]; 
    return this; 
  } 
} 
 
// define product 
const burger = new Burger("hamburger", "small", "onion"); 
 
// adding product to order 
const order = new Menu(burger); 
 
// total bill 
console.log(order.orderBill()); 
 
// remove smth from order 
order.deleteProduct(burger); 
console.log(order);