As of Q1 2015, the Telerik UI for WPF suite uses the standard .NET licensing mechanism, which means that using the controls in design-time may cause Visual Studio to add a license.licx to the solution. This file is used only in design-mode and is not needed for deployment explicitly.
Having this license file is not expected to bring any performance/compilation issues and you can ignore it.
In case your project build fails with errors pointing to the license file, you can delete it and rebuild.
If you are working under source control, once the file is created, it will be detected as a change by your source control system. We would suggest that you check-in this file initially. Then, when it is checked-out again during your work, it is up to you to decide whether to check it in, or discard the changes.