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

[William Kemper]

We only unload classes during a global GC, so it should be safe to reinstate this assertion.

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Commit messages:
 - Assert mark context is complete before unloading classes

Changes: https://git.openjdk.org/shenandoah/pull/488/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=488&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/pull/488.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/488/head:pull/488

PR: https://git.openjdk.org/shenandoah/pull/488