nixos-module: remove nginx recommendedProxySettings, keep recommendedOptimisation

recommendedProxySettings only adds proxy_* directives (proxy_redirect,
proxy_connect_timeout, proxy_set_header Connection, etc.), which only
take effect on locations using proxy_pass. This vhost only ever uses
fastcgi_pass for PHP, so the setting was inert here.

recommendedOptimisation (sendfile/tcp_nopush/tcp_nodelay/keepalive)
does apply regardless of proxy_pass vs fastcgi_pass, so it's kept.
Verified via the full automated test suite (both proxy backends).

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

nixos-module/invoiceninja.nix

@@ -486,7 +486,6 @@ in
 
       recommendedTlsSettings = (if (cfg.hostname == "localhost") then false else true);
       recommendedGzipSettings = true;
-      recommendedProxySettings = true;
       recommendedOptimisation = true;
 
       clientMaxBodySize = cfg.maxUploadSize;