jsapi-util-root: Don't destroy GjsMaybeOwned in notify

Previously the code claimed that it was safe to delete a GjsMaybeOwned
inside its context-destroyed notification callback. However, this is not
true. Remove the comment saying so, and remove the test that tests it.

We did not actually use this feature in the code.

https://bugzilla.gnome.org/show_bug.cgi?id=776966