Remove Redis MultiStore implementation

Based on the discussion in https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/14509, we decided that we should remove the MultiStore implementation. Since we successfully migrated all user sessions to the new instance, there is no need to keep unused code. It will be linked and documented in #1511 (closed).

We also decided not to support self-managed migrations as not necessary. The only downside in case self-managed ever decides to configure a separate session Redis instance, is that currently logged-in users will be logout, and they will need to sign in again.