RFR: 8342119: Shenandoah: Remove extra ShenandoahUpdateRefsClosure

[Roman Kennke]

On Tue, 15 Oct 2024 08:53:00 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> We have a duplicate `ShenandoahUpdateRefsClosure`, which we use in a few places. For bulk GC operations, we have `ShenandoahConcUpdateRefsClosure` and `ShenandoahSTWUpdateRefsClosure` that are semantically similar to that one. We can drop the `ShenandoahUpdateRefsClosure` in favor of these two existing ones. 
> 
> Additional testing:
>  - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`
>  - [ ] Performance tests (no regressions)

Changes look fine. I don't think we would ever do concurrent updates in Thread roots, but yeah let's err on the side of caution. Perf difference should be tiny because there'd be no contention anyway.

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Marked as reviewed by rkennke (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/21517#pullrequestreview-2375298089