RFR: 8373048: Genshen: Remove dead code from Shenandoah

[Kelvin Nilsen]

On Wed, 3 Dec 2025 20:55:09 GMT, Xiaolong Peng <xpeng at openjdk.org> wrote:

> 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
> - [x] GHA

Thanks for this cleanup

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PR Review: https://git.openjdk.org/jdk/pull/28647#pullrequestreview-3537557808