Remove Spree::Adjustment.return_authorization scope

There is no code path that creates adjustments from return
authorizations (since a long time).

This line was introduced without a lot of context in 2013, and there is
nothing in the current codebase that warrants this scope.

core/app/models/spree/adjustment.rb

@@ -48,6 +48,8 @@ class Adjustment < Spree::Base
     scope :is_included, -> { where(included: true) }
     scope :additional, -> { where(included: false) }
 
+    singleton_class.deprecate :return_authorization, deprecator: Spree::Deprecation
+
     extend DisplayMoney
     money_methods :amount