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

[Roman Kennke]

On Thu, 10 Nov 2022 00:47:14 GMT, William Kemper <wkemper at openjdk.org> wrote:

> `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.

Changes look ok. Does it show performance changes/improvements?

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Marked as reviewed by rkennke (Lead).

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