### 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
failed_deps_str = ", ".join(f"{dep.dep_name}: {dep.reason}" for dep in failed_deps)
            flash(
                "Could not queue task instance for execution, dependencies not met: "
                "{}".format(failed_deps_str),
                "error",
            )
            return redirect(origin)
        executor.job_id = "manual"
        executor.start()
        executor.queue_task_instance(
            ti,
            ignore_all_deps=ignore_all_deps,
            ignore_task_deps=ignore_task_deps,
            ignore_ti_state=ignore_ti_state,
        )
        executor.heartbeat()
        flash(f"Sent {ti} to the message queue, it should start any moment now.")
        return redirect(origin)
    @expose('/delete', methods=['POST'])
    @auth.has_access(
        [
            (permissions.ACTION_CAN_DELETE, permissions.RESOURCE_DAG),
        ]
    )
    @action_logging
    def delete(self):
        """Deletes DAG."""
        from airflow.api.common.experimental import delete_dag
        from airflow.exceptions import DagFileExists, DagNotFound
        dag_id = request.values.get('dag_id')