Remove epics list from router

What does this MR do and why?

Following !164191 (merged), it was noticed that the 'Epics' link in the breadcrumbs took the user to the new Work Item based Epics listing page, rather than the legacy page.

This change removes the group level listing page from the router config and replaces the breadcrumb link with a link to the legacy epics listing page.

MR acceptance checklist

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Screenshots or screen recordings

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Before	After

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

1. Visit an epic with work item epics enabled.
2. Click the 'Epics' breadcrumb link.
3. Toggle the work_item_epics_list feature flag
4. On this branch you'll be taken to the legacy epic listing page when the feature flag is disabled
5. On master you'll be taken to the newer epic listing page that uses work items when the feature flag is disabled

Relates to #480084 (closed)