RFR: 8347482: Remove unused field in ParkEvent

[Zhengyu Gu]

A trivial change to remove unused field.

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Commit messages:
 - 8347482: Remove unused field in ParkEvent

Changes: https://git.openjdk.org/jdk/pull/23051/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=23051&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8347482
  Stats: 3 lines in 1 file changed: 0 ins; 3 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/23051.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/23051/head:pull/23051

PR: https://git.openjdk.org/jdk/pull/23051