remove superfluous then

IPython/html/tests/casperjs/util.js

@@ -67,11 +67,9 b' casper.delete_current_notebook = function () {'
 };
 
 casper.wait_for_idle = function () {
-    this.then(function () {
-        this.waitFor(function () {
-            return this.evaluate(function () {
-                return IPython._status == 'idle';
-            });
+    this.waitFor(function () {
+        return this.evaluate(function () {
+            return IPython._status == 'idle';
         });
     });
 };