function removeAt (doc, containerPath, pos) {
  const op = doc.update(containerPath, { type: 'delete', pos })
  if (op && op.diff) {
    return op.diff.val
  }
}
Removes an item from a CHILDREN or CONTAINER property.

@param {Document} doc
@param {string[]} containerPath
@param {number} pos
@returns the id of the removed child