RFR: 8368213: ZGC: Cleanup dead code, unimplemented declarations, unused private fields

[Joel Sikström]

On Mon, 22 Sep 2025 05:58:14 GMT, Axel Boldt-Christmas <aboldtch at openjdk.org> wrote:

> Over the last years refactoring and bug fixes a bit of dead code has slipped through. Clean up some of this. Purposely do not touch unused public declarations with an implementation (most if not all are trivial property checks or simple constructions which have negligible maintenance cost to keep.) nor unused public typedefs.
> 
> The `is_high_usage` helper was rewritten to take a boolean rather than an optional templated print function pointer, as there was only one usage (a lambda which captured nothing).
> 
> Also updated `GCThreadLocalData` size to reflect the changes that has occurred over time. Maybe we can derive this size from all the implementors in the future, but may be to hard to do in a non hack-y way.
> 
> * Testing
>   * Running ZGC test tier 1-5 on Oracle supported platforms.

Marked as reviewed by jsikstro (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/27415#pullrequestreview-3251233319