RFR: 8273071: SeparatorSkin: must remove child on dispose

[Jeanette Winzenburg]

minor skin cleanup issue: SeparatorSkin didn't remove the line it added to the control's children

fix is to override dispose and include the removal
for testing: removed the exclusion of SeparatorSkin from memoryLeakTest - doing so lets it fail/pass before/after this fix

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Commit messages:
 - 8273071: SeparatorSkin: must remove child on dispose

Changes: https://git.openjdk.java.net/jfx/pull/616/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=616&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8273071
  Stats: 14 lines in 2 files changed: 7 ins; 7 del; 0 mod
  Patch: https://git.openjdk.java.net/jfx/pull/616.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/616/head:pull/616

PR: https://git.openjdk.java.net/jfx/pull/616