Remove duplicate implementations in job service and priority resolver #601

Description
This issue is included in this discussion. Under job service and priority resolver, there's a duplicated populateDownstreamDAGs in job/service.go is too tied up with Airflow scheduler. The logic itself is quite similar to buildMultiRootDependencyTree in job/priority_resolver.go.

Acceptance Criteria

• no duplicate implementation between job service and priority resolver

Tech Details

• remove one of the two implementation