RFR: 8364254: Serial: Remove soft ref policy update in WhiteBox FullGC

[Thomas Schatzl]

On Tue, 29 Jul 2025 09:37:26 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

> Simple removing Serial specific logic in `WB_FullGC`, because Serial full-gc handles white-box full-gc already.
> 
> Test: tier1-3

Seems good.

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Marked as reviewed by tschatzl (Reviewer).

PR Review: https://git.openjdk.org/jdk/pull/26528#pullrequestreview-3075898221