LibWeb: Make XMLSerializer GC-allocated

Author: https://github.com/awesomekling
Commit: 233208b640
Pull-request: https://github.com/SerenityOS/serenity/pull/14816
Reviewed-by: https://github.com/ADKaster
Reviewed-by: https://github.com/linusg ✅

Userland/Libraries/LibWeb/Forward.h

@@ -479,7 +479,6 @@ class WorkerNavigatorWrapper;
 class Wrappable;
 class Wrapper;
 class XMLHttpRequestPrototype;
-class XMLSerializerWrapper;
 enum class CanPlayTypeResult;
 enum class CanvasFillRule;
 enum class EndingType;