proxy: do not set Host header when proxying

The default when proxying is $proxy_host.

Proxied apps must used X-Forwarded-Host header to determine the intended
target. I think we overwrote the Host header back in the day because apps
had varied support for this. Ideally, it can be removed across all our configurations.

src/nginxconfig.ejs

@@ -147,7 +147,9 @@ server {
     proxy_read_timeout       3500;
     proxy_connect_timeout    3250;
 
+<% if ( endpoint !== 'external' ) { %>
     proxy_set_header   Host               $host;
+<% } %>
     proxy_set_header   X-Forwarded-For    $remote_addr;
     proxy_set_header   X-Forwarded-Host   $host;
     proxy_set_header   X-Forwarded-Port   $server_port;