Stop unconditionally setting the z-index on sync

This causes a bug where hidden widgets set the z-index of the mask
to their own z-index. The mask z-index is anyway correctly set
previously in _uiSetHostVisibleModal.

You can test this bug manually with this code:

```html
<div id="panel1">
</div>
<div id="panel2">
</div>

<script type="text/javascript">
YUI().use('panel', function (Y) {

        panel1 = new Y.Panel({
            bodyContent: 'This panel opens first, sets the mask zindex correctly, but gets hidden by the second panel rednering',
            width      : 400,
            zIndex     : 6,
            modal      : true,
            visible    : true,
            render     : '#panel1'
        });
        panel2 = new Y.Panel({
            bodyContent: 'This panel, even though it is hidden, sets the mask index, covering panel1',
            zIndex     : 7,
            modal      : true,
            visible    : false,
            render     : '#panel2'
        });

});

</script>
```