**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


airflow/models/dag.py
from airflow.models.dagrun import DagRun
from airflow.models.param import DagParam, ParamsDict
from airflow.models.taskinstance import Context, TaskInstance, TaskInstanceKey, clear_task_instances
from airflow.security import permissions
from airflow.stats import Stats
from airflow.timetables.base import DagRunInfo, DataInterval, TimeRestriction, Timetable
from airflow.timetables.interval import CronDataIntervalTimetable, DeltaDataIntervalTimetable
from airflow.timetables.simple import NullTimetable, OnceTimetable
from airflow.typing_compat import Literal, RePatternType
from airflow.utils import timezone
from airflow.utils.dag_cycle_tester import check_cycle
from airflow.utils.dates import cron_presets, date_range as utils_date_range
from airflow.utils.file import correct_maybe_zipped
from airflow.utils.helpers import validate_key
from airflow.utils.log.logging_mixin import LoggingMixin
from airflow.utils.session import provide_session
from airflow.utils.sqlalchemy import Interval, UtcDateTime, skip_locked, with_row_locks
from airflow.utils.state import DagRunState, State
from airflow.utils.types import DagRunType, EdgeInfoType
if TYPE_CHECKING:
    from airflow.utils.task_group import TaskGroup
log = logging.getLogger(__name__)
DEFAULT_VIEW_PRESETS = ['tree', 'graph', 'duration', 'gantt', 'landing_times']
ORIENTATION_PRESETS = ['LR', 'TB', 'RL', 'BT']
ScheduleIntervalArgNotSet = type("ScheduleIntervalArgNotSet", (), {})
DagStateChangeCallback = Callable[[Context], None]
ScheduleInterval = Union[str, timedelta, relativedelta]
ScheduleIntervalArg = Union[ScheduleInterval, None, Type[ScheduleIntervalArgNotSet]]
DEFAULT_SCHEDULE_INTERVAL = timedelta(days=1)
class InconsistentDataInterval(AirflowException):