gistfile1.cs

string xml = @"
    <root xmlns='http://www.w3.org/TR/html4/' xmlns:h='http://www.w3.org/TR/html4/' xmlns:f='http://www.w3schools.com/furniture'>
    <h:table><h:tr><h:td>Apples</h:td><h:td>Bananas</h:td></h:tr></h:table><f:table><f:name>African Coffee Table</f:name>
    <f:width>80</f:width><f:length>120</f:length></f:table></root>";
XElement x = XElement.Parse(xml);
(from node in x.DescendantsAndSelf()
 where node is XElement
 from attr in node.Attributes()
 where attr.IsNamespaceDeclaration && attr.Name.LocalName == "xmlns"
 select attr).All(attr => { attr.Remove(); return true; });
Console.Out.WriteLine(x.ToString());