RFR: 8253434: Shenandoah: Cleanup ShenandoahRootScanner [v2]

[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 incrementally with one additional commit since the last revision:

  No need to mark on-stack CLD

-------------

Changes:
  - all: https://git.openjdk.java.net/jdk/pull/286/files
  - new: https://git.openjdk.java.net/jdk/pull/286/files/a6d797c3..66492e4f

Webrevs:
 - full: https://webrevs.openjdk.java.net/?repo=jdk&pr=286&range=01
 - incr: https://webrevs.openjdk.java.net/?repo=jdk&pr=286&range=00-01

  Stats: 2 lines in 1 file changed: 0 ins; 1 del; 1 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