nginx: Remove proxy_ssl_certificate settings

I've been running into a segmentation fault in nginx. Weird, because I haven't seen this before, and not changed as far as I can tell.

Anyhow, I've been debugging the process, but I'm failing to pinpoint the exact root cause. Also I've been looking around on the internet, and the closest post I could find is a message on the mailing list from 7 years ago1. They run into a segmentation fault because the same file is used in multiple servers using the ssl_client_certificate directive. That's not something GDK does, but we reuse the same filename in proxy_ssl_certificate directives.

So I did some more digging, from the directives reference documentation2 I didn't learn much, neither did I from the admin guide3. But from that admin guide I see the proxy_ssl_certificate directive is used to set a different file, so I concluded this directive is only needed when the proxy uses a different certificate.