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

[William Kemper]

Clean backport.

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Commit messages:
 - Backport bb7938446b3b6d6729df627cd580957f4a3a2116

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/141/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=141&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-jdk21u/pull/141.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/141/head:pull/141

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/141