RFR: JDK-8284236: Remove java/lang/ref/ReferenceEnqueue.java from ProblemList-Xcomp.txt

[Mandy Chung]

This test failure is no longer reproducible.   It was added to the problem list from JDK-8286368 as a clean up after JEP 425 Virtual Threads integration.  This PR removes it from the problem list.

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Commit messages:
 - JDK-8284236: Remove java/lang/ref/ReferenceEnqueue.java from ProblemList-Xcomp.txt

Changes: https://git.openjdk.org/jdk/pull/12371/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=12371&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8284236
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/12371.diff
  Fetch: git fetch https://git.openjdk.org/jdk pull/12371/head:pull/12371

PR: https://git.openjdk.org/jdk/pull/12371