Removed migration helpes #remove_columns, #add_columns

 * It caused issues when monkeypatching MysqlAdapter due to
   not specifying superclass -- so it depends on the real class
   loading first
 * Also #remove_columns already exists as of Rails3
 * And... #add_columns has never been used by us -- probably because
   usually adding columns requires a bit more info (data type, etc).