LibJS+LibWeb: Replace JS::Utf16String with AK::Utf16String

Author: https://github.com/trflynn89
Commit: a43cb15e81
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/5388
Reviewed-by: https://github.com/shannonbooth ✅

Libraries/LibJS/Forward.h

@@ -219,7 +219,6 @@ struct SourceRange;
 class SourceTextModule;
 class Symbol;
 class Token;
-class Utf16String;
 class VM;
 class PrototypeChainValidity;
 class Value;