Remove work_item_epics_rollout feature flag

What does this MR do and why?

Part of #467686 (closed)

Remove work_item_epics_rollout feature flag

This enables work item epics by default for all users when it's enabled for a group. The feature flag has been enabled in production already and enabled to control epic work items rollout per user.

We still have the work_item_epics feature flag in place which is the rollout per group.

Changelog: added

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