RFR: 8321410: Shenandoah: Remove ShenandoahSuspendibleWorkers flag

[Aleksey Shipilev]

We have flipped `ShenandoahSuspendibleWorkers` to true in [JDK-8305403](https://bugs.openjdk.org/browse/JDK-8305403), and backported it back to 17.0.8. We seem to be in consensus that turning this flag off would likely to cause correctness issues:
  https://github.com/openjdk/jdk/pull/13309#discussion_r1160934484

So we might as well remove this flag altogether.

Additional testing:
 - [ ] `hotspot_gc_shenandoah`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/16984/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=16984&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/jdk/pull/16984.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/16984/head:pull/16984

PR: https://git.openjdk.org/jdk/pull/16984