RFR: 8327677: Parallel: Remove unused ParallelCompactData::clear

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - trivial

Changes: https://git.openjdk.org/jdk/pull/18166/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18166&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327677
  Stats: 7 lines in 2 files changed: 0 ins; 7 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18166.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18166/head:pull/18166

PR: https://git.openjdk.org/jdk/pull/18166