RFR: Enable skip update references

[Roman Kennke]

On Mon, 9 Aug 2021 23:46:29 GMT, William Kemper <wkemper at openjdk.org> wrote:

> This feature of Shenandoah was disabled for generational mode for the reasons described in [PR 50](https://github.com/openjdk/shenandoah/pull/50). Old objects which are no longer reachable and _not_ included in the collection set are filled in at the end of final marking for the old generation so we no longer require the update references phase to perform this task.

Marked as reviewed by rkennke (Lead).

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PR: https://git.openjdk.java.net/shenandoah/pull/54