searchable: refactor searchable_includes.

Pass searchable associations directly to search_attributes instead of
defining them separately in searchable_includes.

app/models/application_record.rb

@@ -93,10 +93,6 @@ class ApplicationRecord < ActiveRecord::Base
 
   concerning :SearchMethods do
     class_methods do
-      def searchable_includes
-        []
-      end
-
       def model_restriction(table)
         table.project(1)
       end