**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
#
#
from unittest import mock
import flask
import pytest
from airflow.dag_processing.processor import DagFileProcessor
from airflow.security import permissions
from airflow.utils.session import create_session
from airflow.utils.state import State
from airflow.www.views import FILTER_STATUS_COOKIE, FILTER_TAGS_COOKIE
from tests.test_utils.api_connexion_utils import create_user
from tests.test_utils.db import clear_db_dags, clear_db_import_errors, clear_db_serialized_dags
from tests.test_utils.www import check_content_in_response, check_content_not_in_response, client_with_login
def clean_db():