RFR: 8241066: Shenandoah: fix or cleanup SH::do_full_collection

[William Kemper]

This `ShenandoahHeap::do_full_collection` will never be called by our implementation of `collect_as_vm_thread`.

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Commit messages:
 - Clarify comment
 - Fix commented out assertion with actual assertion and explanation.

Changes: https://git.openjdk.org/jdk/pull/27853/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27853&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8241066
  Stats: 6 lines in 1 file changed: 3 ins; 0 del; 3 mod
  Patch: https://git.openjdk.org/jdk/pull/27853.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27853/head:pull/27853

PR: https://git.openjdk.org/jdk/pull/27853