Don't pass the Strict-Transport-Security header from upstream.

We set it in the proxy anyway, don't send 2 Strict-Transport-Security
headers.

snippets/proxy-headers.conf

@@ -3,3 +3,4 @@ proxy_set_header    X-Forwarded-Proto $scheme;
 proxy_set_header    X-Forwarded-For $proxy_add_x_forwarded_for;
 proxy_set_header    Host $host;
 proxy_set_header    X-Real-IP $remote_addr;
+proxy_hide_header   Strict-Transport-Security;