**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/dag_processing/test_processor.py
zip_filename = tmpdir / "test_zip.zip"
        with ZipFile(zip_filename, "w") as zip_file:
            zip_file.writestr(TEMP_DAG_FILENAME, PARSEABLE_DAG_FILE_CONTENTS)
        with create_session() as session:
            self._process_file(zip_filename, session)
            import_errors = session.query(errors.ImportError).all()
            assert len(import_errors) == 0
            session.rollback()
    @conf_vars({("core", "dagbag_import_error_tracebacks"): "False"})
    def test_new_import_error_replaces_old(self, tmpdir):
        unparseable_filename = tmpdir / TEMP_DAG_FILENAME
        
        with open(unparseable_filename, 'w') as unparseable_file:
            unparseable_file.writelines(UNPARSEABLE_DAG_FILE_CONTENTS)
        session = settings.Session()
        self._process_file(unparseable_filename, session)
        
        with open(unparseable_filename, 'w') as unparseable_file:
            unparseable_file.writelines(
                PARSEABLE_DAG_FILE_CONTENTS + os.linesep + UNPARSEABLE_DAG_FILE_CONTENTS
            )
        self._process_file(unparseable_filename, session)
        import_errors = session.query(errors.ImportError).all()
        assert len(import_errors) == 1
        import_error = import_errors[0]
        assert import_error.filename == unparseable_filename
        assert import_error.stacktrace == f"invalid syntax ({TEMP_DAG_FILENAME}, line 2)"
        session.rollback()
    def test_remove_error_clears_import_error(self, tmpdir):