AreaComAdminWindow: Fix GstNrInput validation by removing CheckGstNr

Elwig/Helpers/Validator.cs

@@ -589,11 +589,6 @@ namespace Elwig.Helpers {
             return new(true, null);
         }
 
-        public static ValidationResult CheckGstNr(TextBox input, bool required) {
-            // TODO
-            return new(true, "Not implemented yet");
-        }
-
         public static ValidationResult CheckGradatoinOe(TextBox input, bool required) {
             var res = CheckInteger(input, required, 3);
             if (!res.IsValid) {