message: refactor(core): remove eth custom signMessage implementation
  Almost verbatim reimplementation of `BaseCoin.signMessage()`
  Issue:
diff: import { CoinFamily, BaseCoin as StaticsBaseCoin } from '@bitgo/statics';
  import { getBuilder, Eth } from '@bitgo/account-lib';
  import * as Bluebird from 'bluebird';
  -import * as bitcoinMessage from 'bitcoinjs-message';
  import * as bitgoUtxoLib from '@bitgo/utxo-lib';
  import { randomBytes } from 'crypto';
  @@ -172,14 +171,6 @@ export abstract class AbstractEthLikeCoin extends BaseCoin {
  };
  }
  - async signMessage(key: { prv: string }, message: string | Buffer, callback?: NodeCallback<Buffer>): Bluebird<Buffer> {
  - const privateKey = bitgoUtxoLib.HDNode.fromBase58(key.prv).getKey();
  - const privateKeyBuffer = privateKey.d.toBuffer(32);
  - const isCompressed = privateKey.compressed;
  - const prefix = bitgoUtxoLib.networks.bitcoin.messagePrefix;
  - return bitcoinMessage.sign(message, privateKeyBuffer, isCompressed, prefix);
  - }
  -
  isValidPub(pub: string): boolean {
  let valid = true;
  try {
  