Remove chained tokio channels in validate endpoints

[intarga]

blocked on async iterator stabilization

[intarga]

This might be unnecessary now. We can just remove the inner channel in schedule tests. The original idea behind the channel was that it was best to return flags ASAP, and the channel allowed us to do that without waiting for later tests to finish. Now I'm not so convinced of its utility. Since we won't run the combi check to generate end user flags until the full pipeline is finished, it doesn't seem like the individual flags have any use before that point.