RFR: 8338763: GenShen: Global GC should not swap remembered sets for the verifier

[William Kemper]

It seems that subsequent improvements to remembered set verification have made this change unnecessary.

## Testing
GHA, internal pipelines and local dacapo runs

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Commit messages:
 - Remove vestigial comment
 - Do not swap remembered set for global collections

Changes: https://git.openjdk.org/shenandoah/pull/485/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=485&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338763
  Stats: 5 lines in 1 file changed: 0 ins; 3 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/485.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/485/head:pull/485

PR: https://git.openjdk.org/shenandoah/pull/485