Remove attributes from XElement

Please consider this XElement:

<MySerializeClass xmlns:xsi="http://ift.tt/ra1lAU" xmlns:xsd="http://ift.tt/tphNwY">
    <F1>1</F1>
    <F2>2</F2>
    <F3>nima</F3>
</MySerializeClass>

I want to delete xmlns:xsi and xmlns:xsd from above XML. I wrote this code but it does not work:

 XAttribute attr = xml.Attribute("xmlns:xsi");
 attr.Remove();

I got this error:

Additional information: The ':' character, hexadecimal value 0x3A, cannot be included in a name.

How I can delete above attributes?