js: Don't bypass our cache when reloading CSS

The timestamp previously added, forces the browser to ignore
its own cache and thus it doesn't transmit the `If-None-Match`
header. We then re-calculate the the CSS possibly for no reason.
If there is a re-calculation indeed required it should be still
done now since the ETag transmitted by the browser is outdated.

refs #4340

public/js/icinga/ui.js

@@ -111,10 +111,7 @@
                 var $oldLink = $(this);
                 if ($oldLink.hasAttr('type') && $oldLink.attr('type').indexOf('css') > -1) {
                     var base = location.protocol + '//' + location.host;
-                    var url = icinga.utils.addUrlParams(
-                        $(this).attr('href'),
-                        { id: new Date().getTime() }
-                    );
+                    var url = $(this).attr('href');
 
                     var $newLink = $oldLink.clone().attr(
                         'href',