RFR: 8338477: GenShen: Cleanup generational heap

[William Kemper]

Clean

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Commit messages:
 - Backport 1ed7586be1c5271e7a223f1cd14effd2a83b0b84

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/86/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=86&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338477
  Stats: 231 lines in 2 files changed: 96 ins; 96 del; 39 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/86.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/86/head:pull/86

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/86