followup to Bug 431558 - implement preventive maintenance for places.sqlite, remove observer on test finish, r=sdwilsh

toolkit/components/places/tests/unit/test_preventive_maintenance.js

@@ -1101,10 +1101,12 @@ let observer = {
         current_test.setup();
         PlacesDBUtils.maintenanceOnIdle();
       }
-      else
+      else {
+        os.removeObserver(this, FINISHED_MAINTANANCE_NOTIFICATION_TOPIC);
         do_test_finished();
       }
     }
+  }
 }
 os.addObserver(observer, FINISHED_MAINTANANCE_NOTIFICATION_TOPIC, false);