Remove epic legacy new page

What does this MR do and why?

Removes the code to render the legacy new epic page. This is part of the work to remove the work_item_epics feature flag completely.

It also removes some of the dependency with the namespace_level_work_items FF.

• Feature Flag Rollout Issue: #468310 (closed)
• Issue to remove the old code: &17318 (closed)

References

Screenshots or screen recordings

In the "with FF disabled" recording it opened the modal on the second click - this was likely due to a caching of the FF, but shows that it still worked.

With FF enabled	With FF disabled
with_ff	without_ff

How to set up and validate locally

Try it with work_item_epics enabled and disabled:

1. Go to the /epics page on a group
2. Click on the "New" button and create an epic

If a modal pops up when clicking the "New" button, also try to go on the /epics/new page and create an epic.

MR acceptance checklist

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