object: remove ObjectInstance from disassociate_list if object is finalized

Closes: #23 (closed)

After some debugging, instead of reverting commit 05810c9a, I noticed we could just do this.

However, since the combo of

priv->keep_alive.reset();
dissociate_list_remove(priv);

Is done multiple times, if you prefer it can be moved to a single function or adding an unrooting notify to GjsMaybeOwned, if you don't mind some more refactory, otherwise this should be enough.