RFR: 8287734: Shenandoah: Consolidate marking closures

[Zhengyu Gu]

Shenandoah used to have different marking closures that mark metadata or not. With Loom, marking closures always mark metadata, so they can be consolidated.

Test:

- [x] hotspot_gc_shenandoah
- [x] jdk_loom + ShenandoahGC
- [x] hotspot_loom + ShenandoahGC

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Commit messages:
 - Update
 - v0

Changes: https://git.openjdk.java.net/jdk/pull/9000/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=9000&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8287734
  Stats: 46 lines in 2 files changed: 0 ins; 39 del; 7 mod
  Patch: https://git.openjdk.java.net/jdk/pull/9000.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/9000/head:pull/9000

PR: https://git.openjdk.java.net/jdk/pull/9000