`Format::SignatureDeserializationError` is unused

I was trying to identify which errors would be raised if a Biscuit was signed using a malicious private key, and realized `error::Format::SignatureDeserializationError` is used nowhere.

I don’t have the time to remove it myself and make a PR right now but I figured you could do it in a batch next time you work on the lib 😉

(I’m on `74cbf2e`)