RFR: 8240287: SelectedItemsReadOnlyObservableList cleanup: remove unused code.

[Ambarish Rapte]

The below member variables and method of class SelectedItemsReadOnlyObservableList are not required anymore.
Variables: itemsList, itemsListChanged, itemsListChange, itemsListListener.
Method: setItemsList

These members were added when this class was created for [JDK-8154216](https://bugs.openjdk.java.net/browse/JDK-8154216).
But after the fix for [JDK-8152396](https://bugs.openjdk.java.net/browse/JDK-8152396), these class members are not required.
This change also fixes the leak issue : [JDK-8237926](https://bugs.openjdk.java.net/browse/JDK-8237926)

Verification:
No build and test failures.

-------------

Commits:
 - c859a929: [WIP]8240287: SelectedItemsReadOnlyObservableList cleanup: remove unused code.

Changes: https://git.openjdk.java.net/jfx/pull/132/files
 Webrev: https://webrevs.openjdk.java.net/jfx/132/webrev.00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8240287
  Stats: 37 lines in 3 files changed: 0 ins; 37 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/132.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/132/head:pull/132

PR: https://git.openjdk.java.net/jfx/pull/132