Remove the frecent_namespaces_suggestions feature flag

What does this MR do and why?

Remove the frecent_namespaces_suggestions feature flag

This removes the frecent_namespaces_suggestions feature flag and effectively enables the new server-side frecent namespaces suggestions.

Changelog: changed

Screenshots or screen recordings

frecent_items

How to set up and validate locally

1. In any page, open the global search modal (/ keyboard shortcut).
   • Notice that the Frequently visited projects and Frequently visited groups load items.
   • When the query resolves, you should see a list of namespaces you have frequently/recently (frecently) interacted with.
   • There should be more or less the same items as with the :frecent_namespaces_suggestions feature flag disabled, but you might notice some differences as the scoring algorithm boosts most recent visits.
   • If, for some reason¹ you loose access to any namespace in the list, it should not show up anymore the next time frecent items are loaded.
   • You cannot manually remove a project from the list. The ability to remove a project was previously introduced as a workaround to some namespaces remaining in the list even though they had become unavailable (see previous point). This is now all handled server-side.
   • Note that your frecent items persist across browsers/devices since visits are now tracked server-side instead of client-side in the local storage.

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 #428362 (closed)