RFR: 8338444: Shenandoah: Remove ShenandoahHumongousThreshold tunable

[William Kemper]

Not clean, but same resolution as `shenandoah/master`.

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Commit messages:
 - 8338444: Shenandoah: Remove ShenandoahHumongousThreshold tunable

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/121/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=121&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338444
  Stats: 327 lines in 11 files changed: 4 ins; 314 del; 9 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/121.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/121/head:pull/121

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/121