RFR: 8278767: Shenandoah: Remove unused ShenandoahRootScanner

[Zhengyu Gu]

Please review this trivial patch that removes unused ShenandoahRootScanner class.

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Commit messages:
 - 8278767: Shenandoah: Remove unused ShenandoahRootScanner

Changes: https://git.openjdk.java.net/jdk/pull/6832/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=6832&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8278767
  Stats: 37 lines in 2 files changed: 0 ins; 37 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/6832.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/6832/head:pull/6832

PR: https://git.openjdk.java.net/jdk/pull/6832