### 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
}
class XComModelView(AirflowModelView):
    """View to show records from XCom table"""
    route_base = '/xcom'
    list_title = 'List XComs'
    datamodel = AirflowModelView.CustomSQLAInterface(XCom)
    class_permission_name = permissions.RESOURCE_XCOM
    method_permission_name = {
        'list': 'read',
        'delete': 'delete',
        'action_muldelete': 'delete',
    }
    base_permissions = [
        permissions.ACTION_CAN_CREATE,
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_DELETE,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    search_columns = ['key', 'value', 'timestamp', 'execution_date', 'task_id', 'dag_id']
    list_columns = ['key', 'value', 'timestamp', 'execution_date', 'task_id', 'dag_id']
    base_order = ('execution_date', 'desc')
    base_filters = [['dag_id', DagFilter, lambda: []]]
    formatters_columns = {
        'task_id': wwwutils.task_instance_link,
        'execution_date': wwwutils.datetime_f('execution_date'),
        'timestamp': wwwutils.datetime_f('timestamp'),
        'dag_id': wwwutils.dag_link,
    }
    @action('muldelete', 'Delete', "Are you sure you want to delete selected records?", single=False)
    def action_muldelete(self, items):