Remove all code relating to shadow trees, insertion points, shadow
boundaries, traversing composed trees, distribution, template
documents, custom elements, registering elements, element registries,
element factories, shadow roots, etc.
Remove the following features from the IDLs and from the binding
generators: CustomElementCallbacks, Reflect*, EventHandler.
Remove the CSS custom pseudo-element concept, since we no longer have
a UA style sheet worth talking about, no longer have shadow trees or
custom elements, no longer use pseudo-elements, and generally
therefore don't use this code at all.
