Do not try to display discarded products/variants.

[oneiros]

First of all thanks for the wonderful plugin. We have been evaluating it and liked it a lot.

But we found one issue: When we delete a product in the admin backend that is still stored in a wishlist, there is an exception when trying to display that wishlist.

The reason is, that the product is not actually destroyed, but there is a deleted_at timestamp set. And there is a default scope for both Spree::Product and Spree::Variant that only queries records where deleted_at is NULL. That is why the variant stored in the wishlist can no longer be accessed.

The changes in this PR fix the issue for us. Of course, if you prefer another way to fix it, that is totally fine as well.

Thanks again and keep up the good work!