Remove implicit job deduplication feature flags

What does this MR do?

We added these as a safety net, but:

1. We've never used them.
2. There's an implicit feature flag per idempotent worker.
3. Idempotency and job deduplication are now well established in our codebase.

So we don't need them any more.

For gitlab-com/gl-infra/scalability#637 (closed).