message: remove unused SigningKey
diff: @@ -6,17 +6,11 @@ use crate::{
  };
  use chain_core::mempack::{ReadBuf, ReadError, Readable};
  use chain_core::property;
  -use chain_crypto::{Ed25519, Ed25519Extended, PublicKey, SecretKey};
  +use chain_crypto::{Ed25519, PublicKey};
  use std::sync::Arc;
  pub type BftVerificationAlg = Ed25519;
  -/// BFT Leader signing key
  -///
  -/// Both Ed25519Extended and Ed25519 are valid here, but there's
  -/// no way to express this without an enum
  -pub type SigningKey = SecretKey<Ed25519Extended>;
  -
  #[derive(Debug, Clone, PartialEq, Eq, Hash)]
  pub struct LeaderId(pub(crate) PublicKey<BftVerificationAlg>);
  @@ -108,6 +102,7 @@ impl From<PublicKey<BftVerificationAlg>> for LeaderId {
  #[cfg(test)]
  pub mod test {
  use super::*;
  + use chain_crypto::SecretKey;
  use quickcheck::{Arbitrary, Gen};
  impl Arbitrary for LeaderId {
  