Remove cluster addon for dashboard

Dashboard cluster addon had been removed already.
kubernetes/kubernetes#107481

Author: shu-mutou

sig-ui/charter.md

@@ -9,7 +9,6 @@ SIG-UI covers GUI-related aspects of the Kubernetes project. Efforts are centere
 #### Code, Binaries and Services
 
 - [Kubernetes Dashboard](https://github.com/kubernetes/dashboard)
-- [Dashboard Cluster Addon](https://github.com/kubernetes/kubernetes/tree/master/cluster/addons/dashboard)
 - [Dashboard Metrics Scraper](https://github.com/kubernetes-sigs/dashboard-metrics-scraper)
 
 #### Cross-cutting and Externally Facing Processes