RFR: 8351892: GenShen: Remove enforcement of generation sizes

[William Kemper]

* The option to configure minimum and maximum sizes for the young generation have been combined into `ShenandoahInitYoungPercentage`.
* The remaining functionality in `shGenerationSizer` wasn't enough to warrant being its own class, so the functionality was rolled into `shGenerationalHeap`.

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Commit messages:
 - Stop enforcing young/old generation sizes.

Changes: https://git.openjdk.org/jdk/pull/24268/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=24268&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351892
  Stats: 395 lines in 11 files changed: 57 ins; 315 del; 23 mod
  Patch: https://git.openjdk.org/jdk/pull/24268.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/24268/head:pull/24268

PR: https://git.openjdk.org/jdk/pull/24268