Introduce [Abstract] to mark non-leaf interfaces

Some interfaces like Node, CharacterData and HTMLTableCellElement are never
instantiated directly, only their descendant interfaces are. Those are marked
with [Abstract] to set their type_id to None instead of having dummy values
in the TypeId enums.

components/script/dom/xmlhttprequesteventtarget.rs

@@ -10,8 +10,6 @@ use dom::eventtarget::{EventTarget, EventTargetTypeId};
 
 #[derive(Copy, Clone, PartialEq)]
 pub enum XMLHttpRequestEventTargetTypeId {
-    XMLHttpRequestEventTarget,
-
     XMLHttpRequest,
     XMLHttpRequestUpload,
 }