removeTextListener(regexp) {
    const index = this._textRegexpCallbacks.findIndex((textListener) => {
      return String(textListener.regexp) === String(regexp);
    });
    if (index === -1) {
      return null;
    }
    return this._textRegexpCallbacks.splice(index, 1)[0];
  }
Remove a listener registered with `onText()`.
@param {RegExp} regexp RegExp used previously in `onText()`
@return {Object} deletedListener The removed reply listener if
  found. This object has `regexp` and `callback`
  properties. If not found, returns `null`.