When a node is removed from the DOM tree, its paintable needs to be
removed to ensure that it is not used to obtain sizes that are no
longer valid.
This change enables the ResizeObserver to send a notification if a node
is removed, as it should, because a removed node now has a size of zero
It should be okay to nullify pointers without concerning
parent/sibling/child relationships because the layout and paintable
trees will be rebuilt following any DOM mutation anyway.
