Announcements: Avoid duplicate `id attribute

refs #3762

library/Icinga/Web/Widget/Announcements.php

@@ -36,7 +36,7 @@ class Announcements extends AbstractWidget
         $announcements = $repo->findActive();
         $announcements->applyFilter($acked);
         if ($announcements->hasResult()) {
-            $html = '<ul role="alert" id="announcements">';
+            $html = '<ul role="alert">';
             foreach ($announcements as $announcement) {
                 $ackForm = new AcknowledgeAnnouncementForm();
                 $ackForm->populate(array('hash' => $announcement->hash));