RFR: 8265758: [TestBug] Remove ignored unit test from CustomMenuItemTest

[Ajit Ghaisas]

This PR removes the single ignored test from CustomMenuItemTest.

The test unsetContentButNotNull() was incorrectly asserting on not null content. I have remove this test.
The assert for null content has been added to another test getUnspecifiedContentProperty1().

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Commit messages:
 - remove ignored test

Changes: https://git.openjdk.java.net/jfx/pull/477/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jfx&pr=477&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8265758
  Stats: 10 lines in 1 file changed: 1 ins; 8 del; 1 mod
  Patch: https://git.openjdk.java.net/jfx/pull/477.diff
  Fetch: git fetch https://git.openjdk.java.net/jfx pull/477/head:pull/477

PR: https://git.openjdk.java.net/jfx/pull/477