remove buggy callback whose use is not clear

app/controllers/spree/admin/products_controller_decorator.rb

@@ -1,5 +1,4 @@
 Spree::Admin::ProductsController.class_eval do
-  update.before :set_stores
 alias :spree_update :update
   def update
     store_ids = params[:product][:store_ids]
@@ -9,9 +8,4 @@ def update
     spree_update
   end
 
-  private
-
-  def set_stores
-    @product.store_ids = nil unless params[:product].key? :store_ids
-  end
 end