RFR: 8338473: GenShen: Cleanup access levels, whitespace, typos and unused code

[William Kemper]

Clean backport

-------------

Commit messages:
 - Backport 7cc63abcb28f5e1db9303fb174be382809387901

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/84/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=84&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338473
  Stats: 58 lines in 9 files changed: 3 ins; 45 del; 10 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/84.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/84/head:pull/84

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/84