[jdk21u-dev] RFR: 8320907: Shenandoah: Remove ShenandoahSelfFixing flag

[Aleksey Shipilev]

Clean backport to improve Shenandoah reliability and maintainability.

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Commit messages:
 - Backport 43c7f6a673c2fa0b4dbec232e92b621619a98246

Changes: https://git.openjdk.org/jdk21u-dev/pull/149/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk21u-dev&pr=149&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8320907
  Stats: 7 lines in 3 files changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/jdk21u-dev/pull/149.diff
  Fetch: git fetch https://git.openjdk.org/jdk21u-dev.git pull/149/head:pull/149

PR: https://git.openjdk.org/jdk21u-dev/pull/149