RFR: 8325134: Serial: Remove Generation::used_region [v2]

[Thomas Schatzl]

On Fri, 2 Feb 2024 17:49:27 GMT, Albert Mingkun Yang <ayang at openjdk.org> wrote:

>> Trivial removing dead code. The change of moving the definition to `tenuredGeneration.hpp` is needed to work around linking failure.
>
> Albert Mingkun Yang has updated the pull request incrementally with one additional commit since the last revision:
> 
>   review

Marked as reviewed by tschatzl (Reviewer).

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PR Review: https://git.openjdk.org/jdk/pull/17673#pullrequestreview-1860937707