Remove work items group issues list ff

What does this MR do and why?

• This MR removes references to the work_items_group_issues_list feature flag. It should be merged in %18.6, pending the FF becoming default enabled in !207223 (merged).

References

• #562692
• !207223 (merged)

Screenshots or screen recordings

Not provided

How to set up and validate locally

1. Navigate to the group issues list for a particular group
2. Observe that the new group issues list loads (e.g. by looking at the performance bar and noting the requests made to getWorkItemsSlimEE / getWorkItemsFullEE)

MR acceptance checklist

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