### 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 web server times out"""
class AirflowSkipException(AirflowException):
    """Raise when the task should be skipped"""
class AirflowFailException(AirflowException):
    """Raise when the task should be failed without retrying"""
class AirflowDagCycleException(AirflowException):
    """Raise when there is a cycle in Dag definition"""
class AirflowDagDuplicatedIdException(AirflowException):
    """Raise when a Dag's ID is already used by another Dag"""
    def __init__(self, dag_id: str, incoming: str, existing: str) -> None:
        super().__init__(dag_id, incoming, existing)
        self.dag_id = dag_id
        self.incoming = incoming
        self.existing = existing
    def __str__(self) -> str:
        return f"Ignoring DAG {self.dag_id} from {self.incoming} - also found in {self.existing}"
class AirflowClusterPolicyViolation(AirflowException):
    """Raise when there is a violation of a Cluster Policy in Dag definition"""
class AirflowTimetableInvalid(AirflowException):
    """Raise when a DAG has an invalid timetable."""
class DagNotFound(AirflowNotFoundException):
    """Raise when a DAG is not available in the system"""
class DagCodeNotFound(AirflowNotFoundException):
    """Raise when a DAG code is not available in the system"""
class DagRunNotFound(AirflowNotFoundException):
    """Raise when a DAG Run is not available in the system"""
class DagRunAlreadyExists(AirflowBadRequest):
    """Raise when creating a DAG run for DAG which already has DAG run entry"""
class DagFileExists(AirflowBadRequest):