RFR: 8261984: Shenandoah: Remove unused ShenandoahPushWorkerQueuesScope class

[Zhengyu Gu]

Please review this trivial change that removes unused ShenandoahPushWorkerQueuesScope class.

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Commit messages:
 - 8261984: Shenandoah: Remove unused ShenandoahPushWorkerQueuesScope class

Changes: https://git.openjdk.java.net/jdk/pull/2632/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=2632&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8261984
  Stats: 21 lines in 2 files changed: 0 ins; 19 del; 2 mod
  Patch: https://git.openjdk.java.net/jdk/pull/2632.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/2632/head:pull/2632

PR: https://git.openjdk.java.net/jdk/pull/2632