[jdk21u-dev] RFR: 8321410: Shenandoah: Remove ShenandoahSuspendibleWorkers flag

[Aleksey Shipilev]

Clean backport to improve Shenandoah maintainability.

Additional testing:
 - [x] macos-aarch64-server-fastdebug, `hotspot_gc_shenandoah`

-------------

Commit messages:
 - Backport 2830dd2a7d3b933fbddca64ca0ac7a91e7ab0775

Changes: https://git.openjdk.org/jdk21u-dev/pull/221/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=221&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8321410
  Stats: 40 lines in 6 files changed: 0 ins; 33 del; 7 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/221.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/221/head:pull/221

PR: https://git.openjdk.org/jdk21u-dev/pull/221