RFR: 8311821: Simplify ParallelGCThreadsConstraintFunc after CMS removal

[Jenny Shivayogi]

Code oddities reported by SonarCloud after CMS removal
Fix: moving ParallelGCThreadsConstraintFuncParallel to shared code as ParallelGCThreadsConstraintFunc

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Commit messages:
 - moving ParallelGCThreadsConstraintFuncParallel to gc/shared after CMS removal

Changes: https://git.openjdk.org/jdk/pull/14965/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=14965&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8311821
  Stats: 27 lines in 3 files changed: 3 ins; 17 del; 7 mod
  Patch: https://git.openjdk.org/jdk/pull/14965.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/14965/head:pull/14965

PR: https://git.openjdk.org/jdk/pull/14965