RFR: 8368499: GenShen: Do not collect age census during evac when adaptive tenuring is disabled

[William Kemper]

This is a simplification to `ShenandoahEvacTracker` (which is a diagnostic feature). The ability to compute the age census during evacuation was still in the code, even though the (experimental) option to enable it was removed. Removing the rest of the code allows us to take an additional flag-check off the evacuation path and to consolidate duplicated gc stat processing code between `shControlThread` and `shGenerationalControlThread`.

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Commit messages:
 - Decouple age table printing from evac tracking, consolidate duplicated code
 - Do not compute age census during evac when adaptive tenuring is disabled

Changes: https://git.openjdk.org/jdk/pull/27479/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=27479&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8368499
  Stats: 250 lines in 11 files changed: 55 ins; 137 del; 58 mod
  Patch: https://git.openjdk.org/jdk/pull/27479.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/27479/head:pull/27479

PR: https://git.openjdk.org/jdk/pull/27479