RFR: 8341992: GenShen: Fix formatting, remove unreachable code, unused imports and unnecessary comments

[William Kemper]

Improve hygiene for PR for JEP-404.

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Commit messages:
 - Remove unreachable code, unused imports, unnecessary comments, unusual formatting

Changes: https://git.openjdk.org/shenandoah/pull/510/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=510&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8341992
  Stats: 7 lines in 4 files changed: 0 ins; 5 del; 2 mod
  Patch: https://git.openjdk.org/shenandoah/pull/510.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/510/head:pull/510

PR: https://git.openjdk.org/shenandoah/pull/510