Remove DAG.get_num_active_runs
We don't need this function. There's already an almost-identical function on DagRun that we can use, namely DagRun.active_runs_of_dags. Also, make DagRun.active_runs_of_dags private.
- Types of change
- [ ] Dag changes
- [ ] Config changes
- [ ] API changes
- [ ] CLI changes
- [ ] Behaviour changes
- [ ] Plugin changes
- [ ] Dependency changes
- [ ] Code interface changes