The dll is used by Sitefinity Analytics module. The dll System.Windows.dll is included inside Telerik.Sitefinity.Analytics.xap and Telerik.Sitefinity.Silverlight.xap that are loaded by Sitefinity Analytics module and Silverlight upload in Sitefinity 5.x.editions. I don't think removing the reference to System.Windows.dll will break anything. Try removing it and build your app.
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