Changing case type can result in more than one instance of activities specified as max_instances=1 if activity was "with" a contact other than client

This is a followup to CRM-6692.

The code in CRM_Case_XMLProcessor::isActivityPresent has a WHERE clause in the select that checks the activity_target is the client.

So it seems that the obvious fix is to remove that check, except I'm hesitant to do that because it must have been there for a reason. I know that case activities always used to be "with" the client before 3.2, but even then there's no reason I can think of to have this check, so maybe it is there for some other reason? Does it have something to do with multiclient functionality?