sorry - getting confused - conditional is in the 4.2 script
https://github.com/CiviCRM42/civicrm42-core/blob/4.2/CRM/Upgrade/Incremental/php/FourTwo.php#L339

NB - the way this index is added should be our std way of adding indexes (check first then add) IMHO. If that method is used in 4.4 we don't need to remove it from 4.2 script