### 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
]
    )
    @gzipped
    @action_logging
    @provide_session
    def calendar(self, session=None):
        """Get DAG runs as calendar"""
        def _convert_to_date(session, column):
            """Convert column to date."""
            if session.bind.dialect.name == 'mssql':
                return column.cast(Date)
            else:
                return func.date(column)
        dag_id = request.args.get('dag_id')
        dag = current_app.dag_bag.get_dag(dag_id)
        dag_model = DagModel.get_dagmodel(dag_id)
        if not dag:
            flash(f'DAG "{dag_id}" seems to be missing from DagBag.', "error")
            return redirect(url_for('Airflow.index'))
        wwwutils.check_import_errors(dag.fileloc, session)
        root = request.args.get('root')
        if root:
            dag = dag.partial_subset(task_ids_or_regex=root, include_downstream=False, include_upstream=True)
        dag_states = (
            session.query(
                (_convert_to_date(session, DagRun.execution_date)).label('date'),
                DagRun.state,
                func.count('*').label('count'),
            )
            .filter(DagRun.dag_id == dag.dag_id)