Remove DKIM signature after checking it

[link2xt]

DKIM signature is currently checked by OpenDKIM milter attached to smtpd running on port 25. Once it is checked, it is not useful for anything. You cannot change it later because the key may change (see #195). As RFC 6376 states it: "message archiving is not a design goal".

As far as I see from the diagram in https://www.postfix.org/MILTER_README.html#plumbing, cleanup happens after smtpd is done with the message. cleanup is essentially a daemon that puts the message into the queue, so we can remove all unnecessary headers there, including DKIM signature.

This will save users some traffic and server operators some disk space because DKIM headers are quite large compared to other headers.