message: [protocol] Remove unused GETHEADERS_VERSION
  GETHEADERS_VERSION is unused. No need to keep it in the codebase
  for just its historic comment.
diff: @@ -15,9 +15,6 @@ static const int OLD_VERSION = 70014; // peercoin: used to communicate with clie
  //! initial proto version, to be increased after version/verack negotiation
  static const int INIT_PROTO_VERSION = 209;
  -//! In this version, 'getheaders' was introduced.
  -static const int GETHEADERS_VERSION = 31800;
  -
  //! disconnect from peers older than this proto version
  static const int MIN_PEER_PROTO_VERSION = 70016;
  