As a project targeting .NET 3.5 can utilize .NET 2.0 controls with no problems, the Visual Studio ToolBox shows both the .NET 2.0 and .NET 3.5 items.
If you think you do not need the .NET 2.0 items, feel free to remove them. If you end up needing some RadControls items you have previously removed, resetting the VisualStudio ToolBox will re-add them for you.
Please note that removing the .NET 2.0 items will result in you not having any RadControls toolbox items in sites targeting .NET 2.0.
I hope this helps.
the Telerik team