Remove the server_side_frecent_namespaces feature flag

What does this MR do and why?

This removes the server_side_frecent_namespaces feature flag.

Going forward, GitLab instances will start tracking users' visits to projects and groups. There is no user-facing change at the moment as this only enables data-collection. In the next iterations, we'll start leveraging the data to suggest "frecently" (a combination of frequency and recency) visited namespaces to the users.

This new approach will replace the current localStorage-based one which is flawed due to being tied to the current browser and not calculating frecency scores smartly-enough.

Changelog: added

Screenshots or screen recordings

No user-facing change.

How to set up and validate locally

Please refer to the MR that introduced the feature flag if you'd like to test this feature: !123554 (merged).

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 #411879, #417256 (closed)