[Security Solution][Alert details] - finish cleanup of old event_details folder

[PhilippeOberti]

Summary

This work finalizes the effort done to move the code from the common/components.event_details and timelines/components/side_panel folders to the flyout folder.

This PR focuses on:

• cleaning up some unused translations
• removing some unused code
• reorganizing the remaining files in event_details the folder (that are still used in multiple places)

No UI changes should be visible whatsoever! Also, no new code is being added, code is just moved around!

Checklist

• [x] Unit or functional tests were updated or added to match the most common scenarios

https://github.com/elastic/security-team/issues/7462