RFR: 8342081: Shenandoah: Remove extra ShenandoahMarkUpdateRefsSuperClosure

[Aleksey Shipilev]

`ShenandoahMarkUpdateRefsSuperClosure` has only one real subclass, which is used in STW mark. We can collapse the class hierarchy a bit here. 

Additional testing:
 - [x] Linux x86_64 server fastdebug, `hotspot_gc_shenandoah`

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Commit messages:
 - Fix

Changes: https://git.openjdk.org/jdk/pull/21505/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21505&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342081
  Stats: 22 lines in 2 files changed: 2 ins; 11 del; 9 mod
  Patch: https://git.openjdk.org/jdk/pull/21505.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21505/head:pull/21505

PR: https://git.openjdk.org/jdk/pull/21505