chore: remove unused Signature message in hotstuff.proto

[meling]

message Signature {
  oneof Sig {
    ECDSASignature ECDSASig = 1;
    BLS12Signature BLS12Sig = 2;
    EDDSASignature EDDSASig = 3;
  }
}

This is never used in Go code or in the RPCs.