overview: Explicitly reset parent on teradown

Instantiators create objects when they're added to a model, and
deference when when they're removed from the model.

When we explicitly set a parent in onObjectAdded we're creating a second
reference. This does get cleaned up later, but not in the same frame.

This brings us in line to what QQmlRepeater (which works with items)
does internally for items being added and removed.

BUG: 478777


(cherry picked from commit eab90b6a)