RFR: 8338444: Shenandoah: Remove ShenandoahHumongousThreshold tunable

[Aleksey Shipilev]

See the bug for rationale.

Additional testing:
  - [ ] Linux AArch64 server fastdebug, `hotspot_gc_shenandoah`
  - [ ] Linux AArch64 server fastdebug, `all` with `-XX:+UseShenandoahGC`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/20593/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=20593&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338444
  Stats: 246 lines in 10 files changed: 4 ins; 234 del; 8 mod
  Patch: https://git.openjdk.org/jdk/pull/20593.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/20593/head:pull/20593

PR: https://git.openjdk.org/jdk/pull/20593