Remove feature flag for using UNLINK in Redis
What does this MR do?
UNLINK was added in Redis 4. We put this usage of it behind a feature flag in case there was an issue, and also added a fallback for instances using Redis 3.
We no longer support Redis 3 and we haven't needed to disable the feature flag, so we can just simplify this to always use UNLINK.
Related to #213174 (closed) and https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/11018.