RFR: 8338336: GenShen: Cleanup stale TODOs

[William Kemper]

Conflict with https://bugs.openjdk.org/browse/JDK-8332082, left barrier code in 21 unchanged.

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Commit messages:
 - 8338336: GenShen: Cleanup stale TODOs

Changes: https://git.openjdk.org/shenandoah-jdk21u/pull/82/files
  Webrev: https://webrevs.openjdk.org/?repo=shenandoah-jdk21u&pr=82&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8338336
  Stats: 113 lines in 11 files changed: 3 ins; 82 del; 28 mod
  Patch: https://git.openjdk.org/shenandoah-jdk21u/pull/82.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah-jdk21u.git pull/82/head:pull/82

PR: https://git.openjdk.org/shenandoah-jdk21u/pull/82