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/HTMLTextAreaElement.h

@@ -97,7 +97,6 @@ public:
 
     bool check_validity();
     bool report_validity();
-    void set_custom_validity(String const& error);
 
     WebIDL::Long max_length() const;
     WebIDL::ExceptionOr<void> set_max_length(WebIDL::Long);