[AIRFLOW-XXX] Remove duplicate lines from CONTRIBUTING.md (#5830)
[AIRFLOW-XXX] Remove duplicate lines from CONTRIBUTING.md (#5830)

(cherry picked from commit e550afc762d50a84c6313507dc163a99f13fa44c)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow