### 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
"""Prefill the form."""
        try:
            extra = form.data.get('extra')
            if extra is None:
                extra_dictionary = {}
            else:
                extra_dictionary = json.loads(extra)
        except JSONDecodeError:
            extra_dictionary = {}
        if not isinstance(extra_dictionary, dict):
            logging.warning('extra field for %s is not a dictionary', form.data.get('conn_id', '<unknown>'))
            return
        for field in self.extra_fields:
            value = extra_dictionary.get(field, '')
            if value:
                field = getattr(form, field)
                field.data = value
class PluginView(AirflowBaseView):