Delete Attribute of Xml node c#

I have a XML file like this:

<?xml version="1.0" encoding="utf-8"?>
<ApplicationConfiguration xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:ua="http://opcfoundation.org/UA/2008/02/Types.xsd" xmlns="http://opcfoundation.org/UA/SDK/Configuration.xsd">
  <ServerConfiguration>
    <SecurityPolicies>
      <ServerSecurityPolicy>
        <SecurityMode>None_1</SecurityMode>
      </ServerSecurityPolicy>
    </SecurityPolicies>
  </ServerConfiguration>
</ApplicationConfiguration>

What I want is to add more node named ServerSecurityPolicy by code. Then I use this code:

            string docaddress = "D:\\abc.xml";
            XDocument doc = XDocument.Load(docaddress);
            var root = doc.Root;
            var these = root.Descendants().Where(p => p.Name.LocalName == "SecurityPolicies");
            XElement addelement = new XElement("ServerSecurityPolicy");
            addelement.Add(new XElement("SecurityMode", "None_1"));           
            foreach (var elem in these)
            {
                elem.Add(addelement);
            }
            doc.Save(docaddress);

It actually works, but the newly added node is something like this:

      <ServerSecurityPolicy xmlns="">
        <SecurityMode>None_1</SecurityMode>
      </ServerSecurityPolicy>

I don't want the attribute "xmlns", then I try to delete it by something like this:

            var these2 = root.Descendants().Where(p => p.Name.LocalName == "ServerSecurityPolicy");
            foreach (var elem in these2)
            {
                    label2.Text += elem.Attribute("xmlns").Name.LocalName;
                    elem.Attribute("xmlns").Remove();
            }

The label2.Text shows "xmlnsxmlnsxmlsn..." so that I think the elem.Attribute("xmlns") has pointed to the attributes I want to delete, but the Remove() not work. Can you suggest me some ways to delete the attribute or add node without attribute?
Thank you