**Apache Airflow version**: 2.1.0

**What happened**:

We use a python script that reads JSON configs from db to dynamically generate DAGs. Sometimes, when JSON Configs is updated, we expect DAGs to be removed in Airflow.  This doesn't always happen. From my observation, one of the following happens:
1) DAG is removed because the python script no longer generates it.
2) DAG still exists but run history is empty. When trigger the DAG, first task is stuck in `queued` status indefinitely.

**What you expected to happen**:
I expect the right behavior should be 
>1) DAG is removed because the python script no longer generates it.

**How to reproduce it**:
* Create a python script in DAG folder that dynamically generates multiple DAGs
* Execute these dynamically generated DAGs a few times
* Use Airflow Variable to toggle (reduce) the # of DAGs generated
* Examine if number of dynamically generated DAGs in the web UI


tests/www/views/test_views_home.py
resp = client_single_dag.get(page, follow_redirects=True)
    check_content_in_response("Import Errors", resp)
    
    check_content_in_response(f"/{TEST_FILTER_DAG_IDS[0]}.py", resp)
    
    for dag_id in TEST_FILTER_DAG_IDS[1:]:
        check_content_not_in_response(f"/{dag_id}.py", resp)
def test_home_dag_list(working_dags, user_client):
    
    resp = user_client.get('home', follow_redirects=True)
    for dag_id in TEST_FILTER_DAG_IDS:
        check_content_in_response(f"dag_id={dag_id}", resp)
def test_home_dag_list_filtered_singledag_user(working_dags, client_single_dag):
    
    resp = client_single_dag.get('home', follow_redirects=True)
    
    check_content_in_response(f"dag_id={TEST_FILTER_DAG_IDS[0]}", resp)
    
    for dag_id in TEST_FILTER_DAG_IDS[1:]:
        check_content_not_in_response(f"dag_id={dag_id}", resp)
def test_home_robots_header_in_response(user_client):
    
    resp = user_client.get('home', follow_redirects=True)
    assert resp.headers['X-Robots-Tag'] == 'noindex, nofollow'