Remove AppState from Known Issues page

Summary:
Closes https://github.com/facebook/react-native/issues/5682.
Closes https://github.com/facebook/react-native/pull/5704

Reviewed By: svcscm

Differential Revision: D2890937

Pulled By: androidtrunkagent

fb-gh-sync-id: 75760220640a836947ea91e8dc554734dff790b8

docs/KnownIssues.md

@@ -31,7 +31,6 @@ Slider (known as SeekBar)
 #### Modules
 
 ```
-App State
 Camera Roll
 Media
 PushNotificationIOS