RFR: 8351892: GenShen: Remove vestigial young generation sizing options

[Y. Srinivas Ramakrishna]

On Fri, 9 Jan 2026 19:13:31 GMT, William Kemper <wkemper at openjdk.org> wrote:

> GenShen generally tries to keep the young generation as large as possible. The options `ShenandoahMinYoungPercentage` and `ShenandoahMaxYoungPercentage` are no longer used.

Marked as reviewed by ysr (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/29144#pullrequestreview-3667884276