RFR: Improve some defaults and remove unused options for generational mode

[William Kemper]

`ShenandoahPromoteTenuredObjects` and `ShenandoahUsePLABs` are removed.  The default `NewRatio` for generational mode is changed to 1. The default percentage of garbage for regions included in the old generation collection set is lowered to 10.

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Commit messages:
 - Remove declaration of unused ShenandoahUsePLAB
 - Adjust defaults

Changes: https://git.openjdk.org/shenandoah/pull/169/files
 Webrev: https://webrevs.openjdk.org/?repo=shenandoah&pr=169&range=00
  Stats: 37 lines in 3 files changed: 2 ins; 10 del; 25 mod
  Patch: https://git.openjdk.org/shenandoah/pull/169.diff
  Fetch: git fetch https://git.openjdk.org/shenandoah pull/169/head:pull/169

PR: https://git.openjdk.org/shenandoah/pull/169