LibWeb/HTML: Remove uneeded FIXME about use of USVString

An AK::String works fine for a USVString as a USVString is just a more
strict version of DOMString. Maybe we will have a different String type
for it in the future, but for now using an AK::String is fine and we do
not need this FIXME.

Libraries/LibWeb/HTML/ErrorEvent.h

@@ -14,7 +14,7 @@ namespace Web::HTML {
 // https://html.spec.whatwg.org/multipage/webappapis.html#erroreventinit
 struct ErrorEventInit : public DOM::EventInit {
     String message;
-    String filename; // FIXME: This should be a USVString.
+    String filename;
     u32 lineno { 0 };
     u32 colno { 0 };
     JS::Value error { JS::js_null() };