Docs: Remove obsolete P3P policy

P3P is obsolete (https://www.w3.org/TR/P3P11/), therefore the HTTP header
should be removed from the recommended config in the installation docs.

docs/installation/3-http-daemon.md

@@ -32,7 +32,6 @@ server {
         proxy_set_header X-Forwarded-Host $server_name;
         proxy_set_header X-Real-IP $remote_addr;
         proxy_set_header X-Forwarded-Proto $scheme;
-        add_header P3P 'CP="ALL DSP COR PSAa PSDa OUR NOR ONL UNI COM NAV"';
     }
 }
 ```