Remove duplicate method

The method `CheckoutCallbacks#valid_order_line_items?` was a duplicate
of `OrderStockCheck#valid_order_line_items?`.

Apparently, it had been extracted twice:

 * 1d074c2151
 * 06eb98bdf4

But the first commit duplicated the method while the second moved the
original declaration.

app/controllers/concerns/checkout_callbacks.rb

@@ -63,12 +63,6 @@ module CheckoutCallbacks
     end
   end
 
-  def valid_order_line_items?
-    @order.insufficient_stock_lines.empty? &&
-      OrderCycles::DistributedVariantsService.new(@order.order_cycle, @order.distributor).
-        distributes_order_variants?(@order)
-  end
-
   def ensure_order_not_completed
     redirect_to main_app.cart_path if @order.completed?
   end