RFR: 8327477: Parallel: Remove unused ParallelCompactData::_highest_ref

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18139/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18139&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327477
  Stats: 1 line in 1 file changed: 0 ins; 1 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18139.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18139/head:pull/18139

PR: https://git.openjdk.org/jdk/pull/18139