[jdk21u-dev] RFR: 8320877: Shenandoah: Remove ShenandoahUnloadClassesFrequency support

[Aleksey Shipilev]

Clean backport to improve Shenandoah maintainability.

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Commit messages:
 - Backport b65ccff357e2e294b027f693ceb3d25410236a6b

Changes: https://git.openjdk.org/jdk21u-dev/pull/147/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=147&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320877
  Stats: 37 lines in 5 files changed: 0 ins; 34 del; 3 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/147.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/147/head:pull/147

PR: https://git.openjdk.org/jdk21u-dev/pull/147