Remove unnecessary code

db/migrate/20181112103239_drop_default_value_on_status_deployments.rb

 # frozen_string_literal: true
 
 class DropDefaultValueOnStatusDeployments < ActiveRecord::Migration
-  include Gitlab::Database::MigrationHelpers
-
   DOWNTIME = false
   DEPLOYMENT_STATUS_SUCCESS = 2 # Equivalent to Deployment.state_machine.states['success'].value
 
-  disable_ddl_transaction!
-
   def up
     change_column_default :deployments, :status, nil
   end