v8binding: Remove dead code of CustomElementConstructor

IDL type definition of CustomElementConstructor in document.idl
is no longer used.  We have the definition in
custom_element_registry.idl.

Change-Id: I88b44330c083c337a91a3c90ec4fe3056c688037
Reviewed-on: https://chromium-review.googlesource.com/c/1437035Reviewed-by: Kentaro Hara <haraken@chromium.org>
Reviewed-by: Hitoshi Yoshida <peria@chromium.org>
Commit-Queue: Yuki Shiino <yukishiino@chromium.org>
Cr-Commit-Position: refs/heads/master@{#626433}

third_party/blink/renderer/core/dom/document.idl

@@ -18,8 +18,6 @@
  * Boston, MA 02110-1301, USA.
  */
 
-[Custom] callback CustomElementConstructor = Element ();
-
 // https://html.spec.whatwg.org/C/#the-document-object
 enum DocumentReadyState { "loading", "interactive", "complete" };