method mentioned by Andrey Murzov is actually an IList extension method residing in Telerik.Windows.Controls.Map.
/// Holds extension methods for <see cref="IList"/> interface.
/// Removes all items from the list.
/// As opposed to Clear method raise CollectionChanged event with action Remove and OldItems property set.
/// <param name="list">List to remove objects from.</param>
So probably you only need to add the namespace with using in the top section of your file.
Do you need help with upgrading your AJAX, WPF or WinForms project? Check the Telerik API Analyzer
and share your thoughts.