RFR: 8270839: Remove deprecated implementation methods from Scene

[Kevin Rushforth]

This removes following implementation methods in Scene by making them package-scope. They were previously deprecated for removal by [JDK-8270246](https://bugs.openjdk.java.net/browse/JDK-8270246):


    public void disposePeer()
    public void enableInputMethodEvents(boolean enable)
    public void processKeyEvent(KeyEvent e) 


One of the test harness classes was calling `Scene::processKeyEvent` directly, so I changed it to call it via `SceneHelper`.

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Commit messages:
 - 8270839: Remove deprecated implementation methods from Scene

Changes: https://git.openjdk.java.net/jfx/pull/579/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=579&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8270839
  Stats: 25 lines in 2 files changed: 2 ins; 17 del; 6 mod
  Patch: https://git.openjdk.java.net/jfx/pull/579.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/579/head:pull/579

PR: https://git.openjdk.java.net/jfx/pull/579