deregister(node, inertRoot) {
    const inertNode = this._managedNodes.get(node);
    if (!inertNode)
      return null;

    inertNode.removeInertRoot(inertRoot);
    if (inertNode.destroyed)
      this._managedNodes.delete(node);

    return inertNode;
  }
De-register the given InertRoot as managing the given inert node.
Removes the inert root from the InertNode's set of managing inert roots, and remove the inert
node from the InertManager's set of managed nodes if it is destroyed.
If the node is not currently managed, this is essentially a no-op.
@param {Node} node
@param {InertRoot} inertRoot
@return {InertNode?} The potentially destroyed InertNode associated with this node, if any.