Remove forcing legacy epic view

What does this MR do and why?

Removes the force_legacy_view=true param on epics. This removes the ability to render the legacy view without changing the feature flag.

References

Screenshots or screen recordings

Before	After

How to set up and validate locally

1. Go to an epic of your choice
2. Append ?force_legacy_view=true to the URL, e.g. http://gdk.test:3000/groups/gitlab/-/epics/6?force_legacy_view=true
3. Before: it rendered the old epic view
4. After: keeps rendering the work item view

MR acceptance checklist

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