RFR: 8351892: GenShen: Remove vestigial young generation sizing options

[William Kemper]

GenShen generally tries to keep the young generation as large as possible. The options `ShenandoahMinYoungPercentage` and `ShenandoahMaxYoungPercentage` are no longer used.

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Commit messages:
 - Remove vestigial young gen sizing options

Changes: https://git.openjdk.org/jdk/pull/29144/files
  Webrev: https://webrevs.openjdk.org/?repo=jdk&pr=29144&range=00
  Issue: https://bugs.openjdk.org/browse/JDK-8351892
  Stats: 23 lines in 2 files changed: 0 ins; 23 del; 0 mod
  Patch: https://git.openjdk.org/jdk/pull/29144.diff
  Fetch: git fetch https://git.openjdk.org/jdk.git pull/29144/head:pull/29144

PR: https://git.openjdk.org/jdk/pull/29144