terminal: signal object removal during sysview_context_stop()

terminal: signal object removal during sysview_context_stop()

Now that we no longer propagate callback return values, we can safely call
into user-callbacks during sysview_context_stop(). This way, users can
rely on all objects to be removed via callbacks (except if they failed
during object creation). This avoids duplicating any object hashtables on
the users' side and reduces memory consumption.