combineLeftIntoRightSubtree(node) {
    if (node.right) {
      const leftmost = this.getLeftmost(node.right);
      leftmost.setLeftAndUpdateParent(node.left);
      return node.right;
    }
    return node.left;
  }
Combine left into right children into one subtree without given parent node.

@example combineLeftIntoRightSubtree(30)

     30*                             40
   /     \                          /  \
  10      40      combined        35   50
    \    /  \    ---------->     /
    15  35   50                 10
                                  \
                                   15

It takes node 30 left subtree (10 and 15) and put it in the
leftmost node of the right subtree (40, 35, 50).

@param {BinaryTreeNode} node
@returns {BinaryTreeNode} combined subtree