RFR: 8333554: Parallel: Remove unused PSParallelCompact::is_in

[Albert Mingkun Yang]

Trivial removing dead code.

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Commit messages:
 - pgc-remove-is-in

Changes: https://git.openjdk.org/jdk/pull/19546/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19546&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8333554
  Stats: 19 lines in 3 files changed: 1 ins; 18 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/19546.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19546/head:pull/19546

PR: https://git.openjdk.org/jdk/pull/19546