message: fix: remove redundant .then()
diff: @@ -526,10 +526,9 @@ export class Subscriber extends EventEmitter {
  const ackTimeSeconds = (Date.now() - message.received) / 1000;
  this.updateAckDeadline(ackTimeSeconds);
  - // Ignore this in this version of the method (but hook then/catch
  + // Ignore this in this version of the method (but hook catch
  // to avoid unhandled exceptions).
  const resultPromise = this._acks.add(message);
  - resultPromise.then(() => {});
  resultPromise.catch(() => {});
  await this._acks.onFlush();
  @@ -606,7 +605,6 @@ export class Subscriber extends EventEmitter {
  const startTime = Date.now();
  const responsePromise = this._modAcks.add(message, deadline);
  - responsePromise.then(() => {});
  responsePromise.catch(() => {});
  await this._modAcks.onFlush();
  