Disable BGSAVE for redis-registry-cache

In #2097 (comment 1237338976) Sean noticed that BGSAVE is incorrectly enabled for registry-cache.

This happens in our configuration in tanka here: https://gitlab.com/gitlab-com/gl-infra/k8s-workloads/tanka-deployments/blob/088072e349fe587d0cbf4568399d1767307a12a7/environments/redis/values.jsonnet#L8, we should change this to save "".

The instance is used in production, but doesn't have a lot of traffic yet, so the impact of these saves is small.