### 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
)
    @action_logging
    @provide_session
    def gantt(self, session=None):
        """Show GANTT chart."""
        dag_id = request.args.get('dag_id')
        dag = current_app.dag_bag.get_dag(dag_id)
        dag_model = DagModel.get_dagmodel(dag_id)
        root = request.args.get('root')
        if root:
            dag = dag.partial_subset(task_ids_or_regex=root, include_upstream=True, include_downstream=False)
        wwwutils.check_import_errors(dag.fileloc, session)
        dt_nr_dr_data = get_date_time_num_runs_dag_runs_form_data(request, session, dag)
        dttm = dt_nr_dr_data['dttm']
        form = DateTimeWithNumRunsWithDagRunsForm(data=dt_nr_dr_data)
        form.execution_date.choices = dt_nr_dr_data['dr_choices']
        tis = [ti for ti in dag.get_task_instances(dttm, dttm) if ti.start_date and ti.state]
        tis = sorted(tis, key=lambda ti: ti.start_date)
        ti_fails = list(
            itertools.chain(
                *(
                    (
                        session.query(TaskFail)
                        .filter(
                            TaskFail.dag_id == ti.dag_id,
                            TaskFail.task_id == ti.task_id,
                            TaskFail.execution_date == ti.execution_date,
                        )
                        .all()
                    )