Remove promotion_code from adjustment_attributes

Adjustments do not have an attribute called 'promotion_code'. The
resulting JSON will always return NULL.

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

@@ -133,7 +133,7 @@ def required_fields_for(model)
 
       @@adjustment_attributes = [
         :id, :source_type, :source_id, :adjustable_type, :adjustable_id,
-        :amount, :label, :promotion_code,
+        :amount, :label,
         :finalized, :eligible, :created_at, :updated_at
       ]