The directory hasn't been touched for 5 years (other than a couple of typo fixes/renaming). The certificates are expired, the build process notes are out of date, especially with the gitian building and everything, Gavin no longer has anything to do with it, etc.

The signing process is all documented (and scripted) elsewhere (e.g. doc/release-process.md and gitian-building.md, contrib/macdeploy and windeploy), this can just be removed

@theuni can confirm if this is okay, we discussed it on IRC for a bit :)