The checkout was holding a lot of responsibility for knowing which kinds of payment gateways are available and how to initiate the process of redirecting to the external payment page (if needed). This was being hidden somewhat by the way the logic was tucked away in services.
PaymentMethod objects now know whether or not they require an external payment process, and know how that process should be started and how to build the required URL.
So we can now *ask* any payment method if it requires external payment processing or not, and *tell* it to start the process and return the relevant URL (if needed).
We set this value to `true` unconditionally in an initializer, and then check the value in various places via Spree::Config. It's never false, and it's not configurable, so we can just drop it and remove the related conditionals. 🔥
