Remove redis MultiStore implementation

What does this MR do and why?

This MR removes MultiStore implementation, since we successfully migrated active sessions to the new Redis::Sessions dedicated instance.

Screenshots or screen recordings

These are strongly recommended to assist reviewers and reduce the time to merge your change.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

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 gitlab-com/gl-infra/scalability#1510