pad.js: in switchToPad() remove the first iframe before loading the pad

This is an old bug: after an import there were always two iframes with
duplicated content, and the last one was simply on top of the old one.

This bug was there since forever, and became evident when the layout was
migrated to Flexbox.

src/static/js/pad.js

@@ -422,6 +422,11 @@ var pad = {
   },
   switchToPad: function(padId)
   {
+    // destroy old pad from DOM
+    // See https://github.com/ether/etherpad-lite/pull/3915
+    // TODO: Check if Destroying is enough and doesn't leave negative stuff
+    // See ace.js "editor.destroy" for a reference of how it was done before
+    $('#editorcontainer').find("iframe")[0].remove();
     var options = document.location.href.split('?')[1];
     var newHref = padId;
     if (typeof options != "undefined" && options != null){