remove bogus own implementation of jQuery :invalid selector, fixes #952

Closes #952

resources/assets/javascripts/jquery-bundle.js

@@ -154,24 +154,6 @@ $.fn.extend({
     }
 });
 
-$.extend($.expr[':'], {
-    invalid: function(elem, index, match) {
-        var invalids = document.querySelectorAll(':invalid'),
-            result = false,
-            len = invalids.length || 0,
-            i;
-
-        for (i = 0; i < len; i += 1) {
-            if (elem === invalids[i]) {
-                result = true;
-                break;
-            }
-        }
-
-        return result;
-    }
-});
-
 $(document).ready(async () => {
     await setLocale();
     STUDIP.ready.trigger('dom');