Integrated: Enable skip update references

[William Kemper]

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.

This pull request has now been integrated.

Changeset: 6ee74582
Author:    William Kemper <wkemper at openjdk.org>
URL:       https://git.openjdk.java.net/shenandoah/commit/6ee7458222bf92540867d66ef6d41f58ac0f1137
Stats:     3 lines in 2 files changed: 0 ins; 1 del; 2 mod

Enable skip update references

Reviewed-by: rkennke

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