Remove ngf from sitemap_index (#169)

Author: nginx-jack

static/sitemap_index.xml

@@ -6,9 +6,6 @@
     <sitemap>
       <loc>https://docs.nginx.com/nginx-ingress-controller/sitemap.xml</loc>
     </sitemap>
-    <sitemap>
-      <loc>https://docs.nginx.com/nginx-gateway-fabric/sitemap.xml</loc>
-    </sitemap>
     <sitemap>
       <loc>https://docs.nginx.com/nginxaas/azure/sitemap.xml</loc>
     </sitemap>