RFR: 8337546: Remove unused GCCause::_adaptive_size_policy

[Kim Barrett]

On Wed, 31 Jul 2024 11:25:50 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Trivial removing an unused gc-cause; it was previously used by Parallel only.

Looks good, and trivial.

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Marked as reviewed by kbarrett (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/20403#pullrequestreview-2210171121