Remove agent from sitemap index (#180)

Author: nginx-jack

static/sitemap_index.xml

@@ -9,9 +9,6 @@
     <sitemap>
       <loc>https://docs.nginx.com/nginxaas/azure/sitemap.xml</loc>
     </sitemap>
-    <sitemap>
-      <loc>https://docs.nginx.com/nginx-agent/sitemap.xml</loc>
-    </sitemap>
     <sitemap>
       <loc>https://unit.nginx.org/sitemap.xml</loc>
     </sitemap>