Userland: Replace VERIFY(is<T>) with verify_cast<T>

Instead of doing a VERIFY(is<T>(x)) and *then* casting it to T, we can
just do the cast right away with verify_cast<T>. :^)

Userland/Libraries/LibWeb/Loader/FrameLoader.cpp

@@ -276,7 +276,6 @@ void FrameLoader::resource_did_load()
 
     if (auto* host_element = browsing_context().host_element()) {
         // FIXME: Perhaps in the future we'll have a better common base class for <frame> and <iframe>
-        VERIFY(is<HTML::HTMLIFrameElement>(*host_element));
         verify_cast<HTML::HTMLIFrameElement>(*host_element).nested_browsing_context_did_load({});
     }