Eliminate proxy.config.http.append_xforwards_header

The configuration proxy.config.http.append_xforwards_header is never used, instead we have proxy.config.http.insert_squid_x_forwarded_for. To avoid confusion, lets remove the (unused) append_xforwards_header configuration. We could argue that the first one is better named, but insert_squid_x_forwarded_for has been the setting to use since incubation, so lets not break everything, right ?