LibWeb: Switch IDL from UseNewAKString to UseDeprecatedAKString

NewAKString is effectively the default for any new IDL interface, so
let's mark this as the default behavior. It also makes it much easier to
figure out whatever interfaces are still left to port over to new AK
String.

Userland/Libraries/LibWeb/IntersectionObserver/IntersectionObserver.idl

@@ -5,7 +5,7 @@
 
 callback IntersectionObserverCallback = undefined (sequence<IntersectionObserverEntry> entries, IntersectionObserver observer);
 
-[Exposed=(Window), UseNewAKString]
+[Exposed=(Window)]
 interface IntersectionObserver {
     constructor(IntersectionObserverCallback callback, optional IntersectionObserverInit options = {});
     readonly attribute (Element or Document)? root;