removeLast() {
    if (!this.last) return null; // Check if list is already empty.
    const tail = this.last;

    this.last = tail.previous;
    if (this.last) {
      this.last.next = null;
    } else { // if list has size zero, then we need to null out first.
      this.first = null;
    }
    this.size -= 1;
    return tail.value;
  }
Removes element to the end of the list.
Similar to Array.pop().
Runtime: O(1)
@returns {any} the last element's value which was removed