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

[William Kemper]

This PR covers a number of small changes to address comments on https://github.com/openjdk/jdk/pull/20395

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Commit messages:
 - Remove unreachable code, revert unintended changes, fix includes

Changes: https://git.openjdk.org/shenandoah/pull/468/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=468&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338341
  Stats: 34 lines in 8 files changed: 1 ins; 26 del; 7 mod
  Patch: https://git.openjdk.org/shenandoah/pull/468.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/468/head:pull/468

PR: https://git.openjdk.org/shenandoah/pull/468