MDL-39332 Formslib: remove z-index from calendar panel

calendar panel.render() sets zIndex to 0, when calendar is used next to filepicker, it overlaps
zIndex should be set by css and not JS, and previously used removeAttr on panel breaks JS on ie8.
So solution is to render panel and then remove z-index from dom

lib/form/yui/dateselector/dateselector.js

@@ -204,8 +204,11 @@ YUI.add('moodle-form-dateselector', function(Y) {
                 bodyContent : Y.Node.create('<div id="dateselector-calendar-content"></div>'),
                 id : 'dateselector-calendar-panel'
             });
-            this.panel.removeAttr('zIndex'); // z-index is set by the theme.
             this.panel.render(document.body);
+            // zIndex is added by panel.render() and is set to 0.
+            // Remove zIndex from panel, as this should be set by CSS. This can be done by removeAttr but
+            // ie8 fails and there is know issue for it.
+            Y.one('#dateselector-calendar-panel').setStyle('zIndex', null);
             this.panel.on('heightChange', this.fix_position, this);
 
             Y.one('#dateselector-calendar-panel').on('click', function(e){e.halt();});