Remove `vue_epics_list` and Epics list legacy code

What does this MR do and why?

The vue_epics_list feature flag has been enabled by default since %14.7, so this MR removes the flag and older implementation of epics list.

Screenshots or screen recordings

NA

How to set up and validate locally

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

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #276189 (closed), #263195 (closed)