### 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 a DAG ID is still in DagBag i.e., DAG file is in DAG folder"""
class DuplicateTaskIdFound(AirflowException):
    """Raise when a Task with duplicate task_id is defined in the same DAG"""
class SerializedDagNotFound(DagNotFound):
    """Raise when DAG is not found in the serialized_dags table in DB"""
class SerializationError(AirflowException):
    """A problem occurred when trying to serialize a DAG"""
class TaskNotFound(AirflowNotFoundException):
    """Raise when a Task is not available in the system"""
class TaskInstanceNotFound(AirflowNotFoundException):
    """Raise when a Task Instance is not available in the system"""
class PoolNotFound(AirflowNotFoundException):
    """Raise when a Pool is not available in the system"""
class NoAvailablePoolSlot(AirflowException):
    """Raise when there is not enough slots in pool"""
class DagConcurrencyLimitReached(AirflowException):
    """Raise when DAG max_active_tasks limit is reached"""
class TaskConcurrencyLimitReached(AirflowException):
    """Raise when task max_active_tasks limit is reached"""
class BackfillUnfinished(AirflowException):
    """
    Raises when not all tasks succeed in backfill.
    :param message: The human-readable description of the exception
    :param ti_status: The information about all task statuses
    """
    def __init__(self, message, ti_status):
        super().__init__(message)
        self.ti_status = ti_status
class FileSyntaxError(NamedTuple):