### Description

The stable REST API seems to be missing the functionality to delete a DAG. 
This would mirror clicking the "Trash Can" on the UI (and could maybe eventually power it)

![image](https://user-images.githubusercontent.com/80706212/131702280-2a8d0d74-6388-4b84-84c1-efabdf640cde.png)


### Use case/motivation

_No response_

### Related issues

_No response_

### Are you willing to submit a PR?

- [ ] Yes I am willing to submit a PR!

### Code of Conduct

- [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)

airflow/exceptions.py
"""Raise when the application or server cannot handle the request"""
    status_code = 400
class AirflowNotFoundException(AirflowException):
    """Raise when the requested object/resource is not available in the system"""
    status_code = 404
class AirflowConfigException(AirflowException):
    """Raise when there is configuration problem"""
class AirflowSensorTimeout(AirflowException):
    """Raise when there is a timeout on sensor polling"""
class AirflowRescheduleException(AirflowException):
    """
    Raise when the task should be re-scheduled at a later time.
    :param reschedule_date: The date when the task should be rescheduled
    :type reschedule_date: datetime.datetime
    """
    def __init__(self, reschedule_date):
        super().__init__()
        self.reschedule_date = reschedule_date
class AirflowSmartSensorException(AirflowException):
    """
    Raise after the task register itself in the smart sensor service
    It should exit without failing a task
    """
class InvalidStatsNameException(AirflowException):
    """Raise when name of the stats is invalid"""
class AirflowTaskTimeout(AirflowException):
    """Raise when the task execution times-out"""
class AirflowWebServerTimeout(AirflowException):