fix atitraining 400 bad request from duplicate proxy headers

nginx sent Host/X-Real-IP/X-Forwarded-For/X-Forwarded-Proto twice
(explicit proxy_set_header plus the included recommended-proxy-headers
file), which Apache rejected as a malformed request due to the
duplicate Host header. Removed the redundant explicit headers.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

nixos/nginx.nix

@@ -96,12 +96,6 @@
 
         locations."/" = {
           proxyPass = "http://127.0.0.1:3015/";
-          extraConfig = ''
-            proxy_set_header Host $host;
-            proxy_set_header X-Real-IP $remote_addr;
-            proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
-            proxy_set_header X-Forwarded-Proto $scheme;
-          '';
         };
       };