Remove search_global_sso_redirect feature flag

What does this MR do and why?

This removes the derisk search_global_sso_redirect feature flag and all associated specs and code

The feature has been enabled globally since 2025-01-31

https://gitlab.slack.com/archives/CB2S7NNDP/p1739210058186689 (internal)

References

Please include cross links to any resources that are relevant to this MR. This will give reviewers and future readers helpful context to give an efficient review of the changes introduced.

• Related to [Feature flag] Rollout of `search_global_sso_re... (#515346 - closed)
• Advanced search does not re-authenticate when S... (#398572 - closed)

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

N/A

How to set up and validate locally

see the original MR for testing instructions

!179114 (merged)