rspamd: fix DKIM signing for subdomains

With the eSLD normalization feature in rspamd subdomains actually use the DKIM key for their parent domain, which simplifies the setup if you serve multiple subdomains.

We however currently create DKIM key pairs for every given domain name, no matter if it is a second-level domain or subdomain for one, so disabling eSLD normalization aligns with the current intention behind our configuration.

In the future it would be nice if we could reuse the parent domain DKIM key for all its subdomains, but that requires some thought on how to achieve that normalization in nixos-mailserver first.