### 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
pk = self._deserialize_pk_if_composite(pk)
        widgets = self._show(pk)
        widgets['show'].template_args['actions'].pop('userinfoedit')
        return self.render_template(
            self.show_template,
            pk=pk,
            title=self.show_title,
            widgets=widgets,
            related_views=self._related_views,
        )
class CustomUserDBModelView(MultiResourceUserMixin, UserDBModelView):