RFR: Eliminate TODO in ShenandoahHeuristics::select_aged_regions

[Aleksey Shipilev]

Simple fix. Code inspection shows the only use is on generational path.

Additional testing: 
 - [x] macos-aarch64-server-fastdebug `hotspot_gc_shenandoah`

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Commit messages:
 - Simple fix

Changes: https://git.openjdk.org/shenandoah/pull/263/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=263&range=00
  Stats: 4 lines in 1 file changed: 0 ins; 3 del; 1 mod
  Patch: https://git.openjdk.org/shenandoah/pull/263.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah.git pull/263/head:pull/263

PR: https://git.openjdk.org/shenandoah/pull/263