Step by step:
1. Create 2 products
2. Create backend
3. Add backend to both products
4. Delete one product

It should delete only the product and backed usage, not the backend which is used by another product.
I tested it through both API and UI.

Dev Notes
See https://github.com/3scale/porta/blob/d248f3221eb3d6dbeb302a433f63b4b80bf5373a/app/models/service.rb#L25