RFR: 8339127: GenShen: Restore completed mark context assertion during class unloading

[William Kemper]

Clean

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Commit messages:
 - Backport c7cfc7fa32fbe90bed39456288c03d74aa2432b9

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/99/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=99&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8339127
  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/99.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/99/head:pull/99

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/99