RFR: 8342079: Shenandoah: Eliminate extra ShenandoahInitMarkRootsClosure

[Aleksey Shipilev]

`ShenandoahInitMarkRootsClosure` in `ShenandoahSTWMark` is semantically similar to the shared `ShenandoahMarkRefsClosure`. It misses subclassing the metadata-visiting superclass, though, and `ShenandoahMarkRefsClosure` does it right. It is cleaner and safer to use the super-class directly, as concurrent mark does it. 

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/21504/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=21504&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8342079
  Stats: 30 lines in 1 file changed: 1 ins; 27 del; 2 mod
  Patch: https://git.openjdk.org/jdk/pull/21504.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/21504/head:pull/21504

PR: https://git.openjdk.org/jdk/pull/21504