delete (node) {
    let values = node.get(this._property)
    if (!isArray(values)) {
      values = [values]
    }
    values.forEach(value => {
      this.index.delete([value, node.id])
    })
  }
Called when a node has been deleted.

Override this in subclasses for customization.

@private
@param {model/data/Node} node