[API] Remove deprecated ApiHelpers::ATTRIBUTES

Please use Spree::Api::Config now.

Ref solidusio#4039

api/app/helpers/spree/api/api_helpers.rb

@@ -3,51 +3,6 @@
 module Spree
   module Api
     module ApiHelpers
-      ATTRIBUTES = [
-        :product_attributes,
-        :product_property_attributes,
-        :variant_attributes,
-        :image_attributes,
-        :option_value_attributes,
-        :order_attributes,
-        :line_item_attributes,
-        :option_type_attributes,
-        :payment_attributes,
-        :payment_method_attributes,
-        :shipment_attributes,
-        :taxonomy_attributes,
-        :taxon_attributes,
-        :address_attributes,
-        :country_attributes,
-        :state_attributes,
-        :adjustment_attributes,
-        :inventory_unit_attributes,
-        :customer_return_attributes,
-        :return_authorization_attributes,
-        :creditcard_attributes,
-        :payment_source_attributes,
-        :user_attributes,
-        :property_attributes,
-        :stock_location_attributes,
-        :stock_movement_attributes,
-        :stock_item_attributes,
-        :promotion_attributes,
-        :store_attributes,
-        :store_credit_history_attributes,
-        :variant_property_attributes
-      ]
-
-      ATTRIBUTES.each do |attribute|
-        define_method attribute do
-          Spree::Api::Config.send(attribute)
-        end
-
-        define_singleton_method attribute do
-          Spree::Deprecation.warn("Please use Spree::Api::Config::#{attribute} instead.")
-          Spree::Api::Config.send(attribute)
-        end
-      end
-
       def required_fields_for(model)
         required_fields = model._validators.select do |_field, validations|
           validations.any? { |validation| validation.is_a?(ActiveModel::Validations::PresenceValidator) }