[css-shadow-parts-1] Remove IDL section.

Author: fergald

css-shadow-parts-1/Overview.bs

@@ -360,18 +360,4 @@ but never match additional <a>shadow-part pseudo-elements</a>.
     ignoring any other labels.
 </div>
 
-Extensions to the {{Element}} Interface {#idl}
-==============================================
-
-<pre class=idl>
-partial interface Element {
-  attribute DOMString part;
-  attribute DOMString exportparts;
-};
-</pre>
-
-The `exportparts` attribute must reflect the `exportparts` content attribute.
-
-The `part` attribute must reflect the `part` content attribute.
-
-Issue(w3c/csswg-drafts#2414): Define these as superglobals in the DOM spec.
+Issue(w3c/csswg-drafts#2414): Define IDL for structured setting and getting of `part` and `exportparts`.