RFR: 8274546: Shenandoah: Remove unused ShenandoahUpdateRootsTask copy

[Yude Lin]

Remove the unused ShenandoahUpdateRootsTask duplicate from shenandoahConcurrentMark.cpp

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Commit messages:
 - 8274546: Shenandoah: Remove unused ShenandoahUpdateRootsTask copy

Changes: https://git.openjdk.java.net/jdk/pull/5770/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=5770&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8274546
  Stats: 27 lines in 1 file changed: 0 ins; 27 del; 0 mod
  Patch: https://git.openjdk.java.net/jdk/pull/5770.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/5770/head:pull/5770

PR: https://git.openjdk.java.net/jdk/pull/5770