removeLast() {
    let current = this.first;
    let target;

    if (current && current.next) {
      while (current && current.next && current.next.next) {
        current = current.next;
      }
      target = current.next;
      current.next = null;
    } else {
      this.first = null;
      target = current;
    }

    if (target) {
      return target.value;
    }
  }
Removes element to the end of the list
similar to Array.pop
Runtime: O(n)