### 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
error_message = (
                'Given execution date, {}, could not be identified '
                'as a date. Example date format: 2015-11-16T14:34:15+00:00'.format(execution_date)
            )
            response = jsonify({'error': error_message})
            response.status_code = 400
            return response
        task_log_reader = TaskLogReader()
        if not task_log_reader.supports_read:
            return jsonify(
                message="Task log handler does not support read logs.",
                error=True,
                metadata={"end_of_log": True},
            )
        ti = (
            session.query(models.TaskInstance)
            .filter(
                models.TaskInstance.dag_id == dag_id,
                models.TaskInstance.task_id == task_id,
                models.TaskInstance.execution_date == execution_date,
            )
            .first()
        )
        if ti is None:
            return jsonify(
                message="*** Task instance did not exist in the DB\n",
                error=True,
                metadata={"end_of_log": True},
            )
        try: