RFR: 8253434: Shenandoah: Cleanup ShenandoahRootScanner [v4]

[Zhengyu Gu]

> After moving CLDG mark to concurrent phase, ShenandoahRootScanner::strong_roots_do() and
> ShenandoahRootScanner::roots_do() are practical the same, let's trim one.
> Test:
>   hotspot_gc_shenandoah

Zhengyu Gu has updated the pull request with a new target base due to a merge or a rebase. The pull request now
contains four commits:

 - Merge master
 - Remove unused CLD closure
 - No need to mark on-stack CLD
 - Trim ShenandoahRootScanner

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Changes: https://git.openjdk.java.net/jdk/pull/286/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=286&range=03
  Stats: 44 lines in 3 files changed: 2 ins; 37 del; 5 mod
  Patch: https://git.openjdk.java.net/jdk/pull/286.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/286/head:pull/286

PR: https://git.openjdk.java.net/jdk/pull/286