RFR: 8331660: Parallel: Cleanup includes in parallelScavangeHeap files

[Zhengyu Gu]

A trivial cleanup. Remove unused includes and moved includes close to use.

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Commit messages:
 - 8331660: Parallel: Cleanup includes in parallelScavangeHeap files

Changes: https://git.openjdk.org/jdk/pull/19089/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=19089&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8331660
  Stats: 17 lines in 8 files changed: 5 ins; 8 del; 4 mod
  Patch: https://git.openjdk.org/jdk/pull/19089.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/19089/head:pull/19089

PR: https://git.openjdk.org/jdk/pull/19089