Removes the element, with everything under it, from the structure tree.
public void Remove()
ISO 32000-2 §14.7.2, §14.7.5.4. The element leaves its parent's K; for it and every element under it, the parent tree
stops listing them (their MCIDs' array entries become null, their objects lose StructParent and its key), and their IDs
leave the ID tree. The marked content in the page content stays; the dictionaries stay objects of the document until a full save
drops unreachable objects.