removeHelper(word, parent = this, index = 0, meta = { stop: 0 }) {
    if (index === word.length) {
      parent.isWord = false;
      if (Object.keys(parent.children)) { meta.stop = index; }
      return true;
    }
    const child = parent.children[word.charAt(index)];
    if (!child) { return false; }
    if (parent.isWord) { meta.stop = index; }
    const found = this.removeHelper(word, child, index + 1, meta);
    // deletes all the nodes beyond `meta.stop`.
    if (found && index >= meta.stop) {
      delete parent.children[word.charAt(index)];
    }
    return found;
  }
Remove word from trie, return true if found, otherwise false.
@param {string} word - The word to remove.
@param {Trie} parent - The parent node.
@param {number} index - The index.
@param {number} meta.stop - Keeps track of the last letter that won't be removed.
@returns {boolean}