Bug 1649198 - Remove unused and undefined nsXULPrototypeDocument::Create, r=bgrins

Differential Revision: https://phabricator.services.mozilla.com/D81564

dom/xul/nsXULPrototypeDocument.h

@@ -32,8 +32,6 @@ class nsXULPrototypePI;
  */
 class nsXULPrototypeDocument final : public nsISerializable {
  public:
-  static nsresult Create(nsIURI* aURI, nsXULPrototypeDocument** aResult);
-
   typedef std::function<void()> Callback;
 
   // nsISupports interface