removing the root element in xml in java

It sounds like you just want to replace the root element with its single child, right? So you want:

XDocument doc = XDocument.Load(@"C:\Users\ADMIN\Pictures\sample.xml");
doc.Root.ReplaceWith(doc.Root.Elements().Single());

That's all you should need to do. Then doc's root element will be <collection>. I've just tried this, and it was fine.

If you really have to load the document as an XElement (why?) you could always just use:

rootElement = rootElement.Elements().Single();

You don't need to "remove" the root element - just navigate to it.