Remove obsolete screenshot.

This screenshot is no longer accurate, and I don't think it's useful
anyway, so I'm deleting it rather than updating it.

Author: DanAlbert

native-activity/README.md

@@ -19,7 +19,3 @@ clear.
 
 [GameActivity]: https://developer.android.com/games/agdk/game-activity
 [NativeActivity]: http://developer.android.com/reference/android/app/NativeActivity.html
-
-## Screenshots
-
-![screenshot](screenshot.png)