message: refactor(core): remove custom v2 signMessage implementation
  Verbatim reimplementation of BaseCoin.signMessage
  Issue:
diff: @@ -2196,16 +2196,4 @@ export abstract class AbstractUtxoCoin extends BaseCoin {
  verifyRecoveryTransaction(txInfo: VerifyRecoveryTransactionOptions): Bluebird<any> {
  return Bluebird.reject(new errors.MethodNotImplementedError());
  }
  -
  - signMessage(key: { prv: string }, message: string | Buffer, callback?: NodeCallback<Buffer>): Bluebird<Buffer> {
  - return co<Buffer>(function* cosignMessage() {
  - const privateKey = bitcoin.HDNode.fromBase58(key.prv).getKey();
  - const privateKeyBuffer = privateKey.d.toBuffer(32);
  - const isCompressed = privateKey.compressed;
  - const prefix = bitcoin.networks.bitcoin.messagePrefix;
  - return bitcoinMessage.sign(message, privateKeyBuffer, isCompressed, prefix);
  - })
  - .call(this)
  - .asCallback(callback);
  - }
  }
  