### 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
"""Formats duration."""
        end_date = self.get('end_date')
        duration = self.get('duration')
        if end_date and duration:
            return timedelta(seconds=duration)
        return None
    formatters_columns = {
        'log_url': log_url_formatter,
        'task_id': wwwutils.task_instance_link,
        'hostname': wwwutils.nobr_f('hostname'),
        'state': wwwutils.state_f,
        'execution_date': wwwutils.datetime_f('execution_date'),
        'start_date': wwwutils.datetime_f('start_date'),
        'end_date': wwwutils.datetime_f('end_date'),
        'queued_dttm': wwwutils.datetime_f('queued_dttm'),
        'dag_id': wwwutils.dag_link,
        'duration': duration_f,
    }
    @provide_session
    @action(
        'clear',
        lazy_gettext('Clear'),
        lazy_gettext(
            'Are you sure you want to clear the state of the selected task'
            ' instance(s) and set their dagruns to the QUEUED state?'
        ),
        single=False,
    )
    def action_clear(self, task_instances, session=None):