Update tor-spec for ed25519 link authentication keys
tor-spec.txt says that the ed25519 link authentication key isn't used:
These are Ed25519 keys:
- A long-term "master identity" key. This key never
changes; it is used only to sign the "signing" key below. It may be
kept offline.
- A medium-term "signing" key. This key is signed by the master identity
key, and must be kept online. A new one should be generated
periodically.
- A short-term "link authentication" key. Not yet used.