Remove `prevent_issue_epic_search` feature flag

What does this MR do and why?

This issue removes the 'prevent_issue_epic_search' feature flag, as it has been enabled for more than one day on production.

MR acceptance checklist

✅

Screenshots or screen recordings

N/A

How to set up and validate locally

Duo Chat should be able to answer all of these with no error:

1. (on an issue page) What is this issue about?
2. (on any page) Can you find the issue where the empty streaming bug was fixed on Duo Chat? [should say it can't search]
3. (on an epic page) What is this epic about?
4. (on any page) Can you find the epic about making Duo Chat more user-friendly? [should say it can't search]

Related to #463698 (closed)