RFR: 8264788: Make SequentialSubTasksDone use-once

Wed Apr 7 08:38:51 UTC 2021

Hi all,

  can I have reviews for this change that simplifies code ahead?

SequentialSubTasksDone has some machinery to reset itself automatically after all tasks were claimed.

This is not used at all and can/should be removed. 

Test: tier1-3

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Commit messages:
 - Initial commit

Changes: https://git.openjdk.java.net/jdk/pull/3372/files
 Webrev: https://webrevs.openjdk.java.net/?repo=jdk&pr=3372&range=00
  Issue: https://bugs.openjdk.java.net/browse/JDK-8264788
  Stats: 86 lines in 5 files changed: 2 ins; 67 del; 17 mod
  Patch: https://git.openjdk.java.net/jdk/pull/3372.diff
  Fetch: git fetch https://git.openjdk.java.net/jdk pull/3372/head:pull/3372

PR: https://git.openjdk.java.net/jdk/pull/3372