don't swallow stock-only xform_ids during case rebuilds

fix of #4880

http://manage.dimagi.com/default.asp?147961

this was introduced by me in dee0551. intrahealth forms don't actually update cases, they just update ledgers, so these were getting cleared from the case.xform_ids property which was messing up the case history view. this shouldn't have any effect on data integrity since that property is only really used for reports.