Remove documentation on removed excluded_entities key (#9442)

Removed in https://github.com/home-assistant/home-assistant-polymer/pull/2754

source/lovelace/yaml-mode.markdown

@@ -46,9 +46,7 @@ resources:
 
 # Optional background for all views. Check https://developer.mozilla.org/en-US/docs/Web/CSS/background for more examples.
 background: center / cover no-repeat url("/background.png") fixed
-# Exclude entities from "Unused entities" view
-excluded_entities:
-  - weblink.router
+
 views:
     # View tab title.
   - title: Example