RFR: 8322204: Parallel: Remove unused _collection_cost_margin_fraction

[Albert Mingkun Yang]

On Fri, 15 Dec 2023 15:18:18 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial removing dead code.

Thanks for the review.

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PR Comment: https://git.openjdk.org/jdk/pull/17124#issuecomment-1860427862