Remove javafx.scene.accessibility.Accessible, Node#getAccessible(), and Scene#getAccessible()

The ideas was first tracked here:
https://wiki.openjdk.java.net/display/OpenJFX/A11y+TODO
-Hide getAccessible() and the Accessible class from public API (IGNORE, see next item)
-Why do we even need the Accessible class? It only delegates to PlatformAccessible. In Node/Scene we always override its methods and actually delegate to their acc*() counter-parts. So why not use the PlatformAccessible directly.