RFR: 8344592: GenShen: Remove unnecessary comments and changes

[William Kemper]

Remove unnecessary comments and reduce granularity of access controls in shConcurrentGC.hpp.

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Commit messages:
 - Decrease granularity of access modifiers
 - Remove unnecessary comments

Changes: https://git.openjdk.org/shenandoah/pull/545/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=545&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344592
  Stats: 32 lines in 3 files changed: 4 ins; 21 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/545.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/545/head:pull/545

PR: https://git.openjdk.org/shenandoah/pull/545