RFR: 8373048: Genshen: Remove dead code from Shenandoah

[Xiaolong Peng]

Trivial PR to remove dead code from Shenandoah. I noticed some dead code in shenandoahFreeSet.cpp when I was working on https://github.com/openjdk/jdk/pull/26171, this PR is to clean up the dead code in shenandoahFreeSet.cpp and some other files, no functional change at all.

### Test
- [x] hotspot_gc_shenandoah
- [ ] GHA

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Commit messages:
 - Removed dead code

Changes: https://git.openjdk.org/jdk/pull/28647/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=28647&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8373048
  Stats: 145 lines in 7 files changed: 0 ins; 143 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/28647.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/28647/head:pull/28647

PR: https://git.openjdk.org/jdk/pull/28647