LINQ to XML でノード削除する。

public void Delete(string id) {
  var fileName = Request.MapPath(DATA_FILE_PATH);

  var xml = XDocument.Load(fileName);
  var result = from item in xml.Descendants("item")
               where (string)item.Element("id") == id
               select item;

  foreach(var item in result.ToList()) item.Remove();

  xml.Save(fileName);
}