refactor: use removeAttribute for onclick rather than seting to null

Per:

   https://developer.mozilla.org/en-US/docs/Web/API/Element/removeAttribute

You should use removeAttribute() instead of setting the attribute
value to null either directly or using setAttribute(). Many attributes
will not behave as expected if you set them to null.

Author: John P. rouillard

html/classhelper.js

@@ -137,7 +137,7 @@ class ClassHelper extends HTMLElement {
 
             // Removing the helpurl click behavior
             this.helpurlScript = this.helpurl.getAttribute("onclick");
-            this.helpurl.setAttribute("onclick", "");
+            this.helpurl.removeAttribute("onclick", "");
             this.helpurl.addEventListener("click", this.preventDefault);
 
             this.helpurlProps = ClassHelper.parseHelpUrlProps(this.helpurl);