[AIRFLOW-2782][Airflow 2782] Removes unused hard-coded dagreD3
[AIRFLOW-2782][Airflow 2782] Removes unused hard-coded dagreD3

Closes #3635 from verdan/AIRFLOW-2782-dagred3-fix

lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow