Remove Redis trace chunks feature flag

Allow configuring a Redis instance for storing trace chunks. For almost every GitLab instance except GitLab.com, all Redis instances are the same instance. The trace chunks REdis is only used in the ci_enable_live_trace feature flag is enabled (the flag is off by default).

If that flag is enabled and the instance was already using the persistent Redis, then the only way to cleanly migrate is to take downtime. We are not providing additional migration support at this time as this is a niche use-case on top of a niche use-case, and so the additional complexity in the application isn't worth it right now.

For gitlab-com/gl-infra/scalability#1131 (closed).