RFR: 8344773: SM cleanup in ForkJoinPool

[Doug Lea]

Complete the SM cleanup in java.util.concurrent that was deferred during other FJP updates

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Commit messages:
 - Merge branch 'openjdk:master' into JDK-8344773
 - Remove SecurityManager-related code; refactor resetThreadLocals

Changes: https://git.openjdk.org/jdk/pull/22324/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=22324&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8344773
  Stats: 132 lines in 2 files changed: 7 ins; 97 del; 28 mod
  Patch: https://git.openjdk.org/jdk/pull/22324.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/22324/head:pull/22324

PR: https://git.openjdk.org/jdk/pull/22324