RFR: 8353471: ZGC: Redundant generation id in ZGeneration

[Joel Sikström]

On Wed, 2 Apr 2025 06:52:49 GMT, Joel Sikström <jsikstro at openjdk.org> wrote:

> The ZGeneration class (and in turn ZGenerationOld and ZGenerationYoung) keeps track of its own ZGenerationId, which means that the generation id does not need to be passed along as an argument when calling internal functions.
> 
> I've removed the id parameter from `ZGeneration::select_relocation_set` in favor of using the member variable `_id`.

Thank you for the reviews!

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PR Comment: https://git.openjdk.org/jdk/pull/24374#issuecomment-2778471557