Do not touch invoice on retraction

aleksis/apps/paweljong/models.py

@@ -288,11 +288,6 @@ class EventRegistration(ExtensibleModel):
     def retract(self):
         # Remove person from group
         self.event.linked_group.members.remove(self.person)
-        # Invoice
-        inv = self.get_invoice()
-        inv.for_object = None
-        inv.for_object_class = None
-        inv.save()
         # Mark registration as retracted
         self.retracted = True
         self.retracted_date = datetime.today()