Refactor shipping_preference

Extract its determination to a separate class, also remove useless CartRepository and use single PU to avoid confusion.

modules/ppcp-subscription/services.php

@@ -29,6 +29,7 @@ return array(
 			$repository,
 			$endpoint,
 			$purchase_unit_factory,
+			$container->get( 'api.factory.shipping-preference' ),
 			$payer_factory,
 			$environment
 		);