The Telerik.Web.Design assembly is only required for the Visual Studio design-time environment. As such, feel free to remove any references from your web.config file.
As for getting the Visual Studio 2010 design-time working without the Telerik.Web.Design.dll, I think that wont be possible. You should ask your customer/employer to associate his license with your Telerik account, so you gain access to the installation package, containing all necessary additional files.
the Telerik team
If you want to get updates on new releases, tips and tricks and sneak peeks at our product labs directly from the developers working on the RadControls for ASP.NET AJAX, subscribe to their blog feed