All: Remove relevanssi plugin

We never enabled it on the new infra for doc sites, and it was also
never enabled on the blogs.

The default search seems good enough on WordPress 6, plus for the
most important sites (jquery.com, jqueryui.com) we use
Typesense/Algolia

Ref jquery/infrastructure-puppet#17.
Ref jquery/infrastructure-puppet#33.

Author: Krinkle

mu-plugins/_loader.php

@@ -1,6 +1,5 @@
 <?php
 
 // Enabled everywhere
-require_once __DIR__ . '/../plugins/relevanssi/relevanssi.php';
 require_once __DIR__ . '/../plugins/disable-emojis/disable-emojis.php';
 require_once __DIR__ . '/../plugins/gilded-wordpress.php';