### 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/www/views.py
origin = get_safe_url(request.values.get('origin'))
        try:
            delete_dag.delete_dag(dag_id)
        except DagNotFound:
            flash(f"DAG with id {dag_id} not found. Cannot delete", 'error')
            return redirect(request.referrer)
        except DagFileExists:
            flash(f"Dag id {dag_id} is still in DagBag. Remove the DAG file first.", 'error')
            return redirect(request.referrer)
        except AirflowException:
            flash(
                f"Cannot delete DAG with id {dag_id} because some task instances of the DAG "
                "are still running. Please mark the  task instances as "
                "failed/succeeded before deleting the DAG",
                "error",
            )
            return redirect(request.referrer)
        flash(f"Deleting DAG with id {dag_id}. May take a couple minutes to fully disappear.")
        
        return redirect(origin)
    @expose('/trigger', methods=['POST', 'GET'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_EDIT, permissions.RESOURCE_DAG),
            (permissions.ACTION_CAN_CREATE, permissions.RESOURCE_DAG_RUN),
        ]
    )
    @action_logging
    @provide_session
    def trigger(self, session=None):