remove unused usings

Author: mikeebowen

src/DocumentFormat.OpenXml.Framework/OpenXmlCompositeElement.cs

@@ -3,13 +3,11 @@
 
 using DocumentFormat.OpenXml.Framework;
 using System;
-using System.Collections;
 using System.Collections.Generic;
 using System.Diagnostics;
 using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 using System.IO;
-using System.Linq;
 using System.Text;
 using System.Xml;