[AIRFLOW-3635] Fix incorrect logic in detele_dag (introduced in PR#4406) (#4445)

Incorrect logic was introduced in PR #4406
(https://github.com/apache/airflow/pull/4406)

This was not found out because the Travis CI was not working as expected.

airflow/api/common/experimental/delete_dag.py

@@ -41,7 +41,7 @@ def delete_dag(dag_id, keep_records_in_log=True):
     if dag is None:
         raise DagNotFound("Dag id {} not found".format(dag_id))
 
-    if dag.fileloc and not os.path.exists(dag.fileloc):
+    if dag.fileloc and os.path.exists(dag.fileloc):
         raise DagFileExists("Dag id {} is still in DagBag. "
                             "Remove the DAG file first: {}".format(dag_id, dag.fileloc))