LibWeb: Implement setCustomValidity IDL attribute

Author: https://github.com/Psychpsyo
Commit: a8004a77bb
Pull-request: https://github.com/LadybirdBrowser/ladybird/pull/3587
Reviewed-by: https://github.com/ADKaster ✅

Libraries/LibWeb/HTML/HTMLInputElement.h

@@ -151,7 +151,6 @@ public:
 
     WebIDL::ExceptionOr<bool> check_validity();
     WebIDL::ExceptionOr<bool> report_validity();
-    void set_custom_validity(String const&);
 
     WebIDL::ExceptionOr<void> show_picker();