RFR: 8253434: Shenandoah: Cleanup ShenandoahRootScanner

[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

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Commit messages:
 - Trim ShenandoahRootScanner

Changes: https://git.openjdk.java.net/jdk/pull/286/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=286&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8253434
  Stats: 48 lines in 3 files changed: 2 ins; 43 del; 3 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