RFR: Eliminate TODO in ShenandoahHeuristics::select_aged_regions

[William Kemper]

On Fri, 21 Apr 2023 15:06:47 GMT, Aleksey Shipilev <shade at openjdk.org> wrote:

> Simple fix. Code inspection shows the only use is on generational path.
> 
> Additional testing: 
>  - [x] macos-aarch64-server-fastdebug `hotspot_gc_shenandoah`

Marked as reviewed by wkemper (Committer).

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PR Review: https://git.openjdk.org/shenandoah/pull/263#pullrequestreview-1396218507