Maybe get rid of log.log_http_headers config

[LukasKalbertodt]

This is a pretty ad-hoc specific config, mainly to debug setting up an auth system. For some time now, we have a fairly flexible filter system for logging, where logging levels can be set per module path. We could just do the header logging in a new dummy module and then remove the config option.

mod header_logging {
    fn log(req: &Request) { ... }
}

It makes the code a bit more meh, but gets rid of one config option by integrating it into a more flexible solution, which is nice. 🤷

[LukasKalbertodt]

Fixed in #1309