Get rid of redundant class cast

1. GraphicsEnvironment.java:109 - class cast is redundant
2. KeyboardFocusManager.java:2655 - class cast is redundant
3. KeyboardFocusManager.java:2985 - class cast is redundant
4. DefaultKeyboardFocusManager.java:816 - class cast is redundant
5. DefaultKeyboardFocusManager.java:1067 - class cast is redundant