RFR: 8338341: GenShen: Cleanup headers, unreachable code and unintentional changes

[William Kemper]

Conflict in file changed on tip, but doesn't exist in 21.

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Commit messages:
 - 8338341: GenShen: Cleanup headers, unreachable code and unintentional changes

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/80/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=80&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338341
  Stats: 33 lines in 7 files changed: 0 ins; 26 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/80.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/80/head:pull/80

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/80