Remove 'relation' from simple scopes definition.

Remove 'relation' from simple scopes definition as it neglects other chain scopes and returns all products.

Steps to reproduce: (In rails console)
-> Get some ActiveRecord::Relation for Spree::Product model, say Spree::Product.limit(2).ascend_by_updated_at

Actual result: Returns all products ascend by updated at
Expected result: Returns 2 products ascend by updated at