### 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
"""Form widget used to display connection"""
    field_behaviours = json.dumps(ProvidersManager().field_behaviours)
class ConnectionModelView(AirflowModelView):
    """View to show records from Connections table"""
    route_base = '/connection'
    datamodel = AirflowModelView.CustomSQLAInterface(Connection)  
    class_permission_name = permissions.RESOURCE_CONNECTION
    method_permission_name = {
        'add': 'create',
        'list': 'read',
        'edit': 'edit',
        'delete': 'delete',
        'action_muldelete': 'delete',
        'action_mulduplicate': 'create',
    }
    base_permissions = [
        permissions.ACTION_CAN_CREATE,
        permissions.ACTION_CAN_READ,
        permissions.ACTION_CAN_EDIT,
        permissions.ACTION_CAN_DELETE,
        permissions.ACTION_CAN_ACCESS_MENU,
    ]
    extra_fields = list(ProvidersManager().connection_form_widgets.keys())
    list_columns = [
        'conn_id',
        'conn_type',