RFR: 8327022: Parallel: Remove experimental dense prefix calculation

[Thomas Schatzl]

Hi all,

  please review this trivial change to remove some experimental dense prefix calculation which has only been included in non-product builds and which results were always (regardless of build) thrown away.

Testing: gha, local compilation

Thanks,
  Thomas

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Commit messages:
 - 8327022

Changes: https://git.openjdk.org/jdk/pull/18057/files
 Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=18057&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8327022
  Stats: 154 lines in 2 files changed: 0 ins; 154 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/18057.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/18057/head:pull/18057

PR: https://git.openjdk.org/jdk/pull/18057