RFR: 8319779: SystemMenu: memory leak due to listener never being removed

[Johan Vos]

A listener was added but never removed.
This patch removes the listener when the menu it links to is cleared. Fix for https://bugs.openjdk.org/browse/JDK-8319779

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Commit messages:
 - A listener was added but never removed.

Changes: https://git.openjdk.org/jfx/pull/1283/files
 Webrev: https://webrevs.openjdk.org/?repo=jfx&pr=1283&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8319779
  Stats: 61 lines in 1 file changed: 40 ins; 19 del; 2 mod
  Patch: https://git.openjdk.org/jfx/pull/1283.diff
  Fetch: git fetch https://git.openjdk.org/jfx.git pull/1283/head:pull/1283

PR: https://git.openjdk.org/jfx/pull/1283