The property system property "calendar" has a couple of issues.
The first is the naming is not consistent and ambiguous - it should be prepended with "assignments." so that it is more easily recognized as what the property is related to.

The other issue is in how this is used. In looking into the property, I could only find one place that the property is pulled - BaseAssignmentService.getCalendar(contextID) (private method) - if the property is missing, then the method tries to find the site's calendar. If the property is set, the method returns null. This method is only used by BaseAssignment.removeAssignmentAndAllReferences(...) in order to remove associated calendar event item. Unless there is a use/reference here that I missed, this property should be marked DEPRECATED in default.sakai.properties and the code should updated so that it doesn't even bother trying to find this property.