Remove unused DdlHelper in ForeignKeyTest

activerecord/test/cases/migration/foreign_key_test.rb

 require "cases/helper"
-require "support/ddl_helper"
 require "support/schema_dumping_helper"
 
 if ActiveRecord::Base.connection.supports_foreign_keys_in_create?
@@ -26,7 +25,6 @@ def test_foreign_keys
   module ActiveRecord
     class Migration
       class ForeignKeyTest < ActiveRecord::TestCase
-        include DdlHelper
         include SchemaDumpingHelper
         include ActiveSupport::Testing::Stream