RFR: 8342119: Shenandoah: Remove extra ShenandoahUpdateRefsClosure

[Aleksey Shipilev]

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)

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21517/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21517&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342119
  Stats: 37 lines in 5 files changed: 6 ins; 24 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/21517.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21517/head:pull/21517

PR: https://git.openjdk.org/jdk/pull/21517