### 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/api_connexion/endpoints/dag_endpoint.py
"""Update the specific DAG"""
    dag = session.query(DagModel).filter(DagModel.dag_id == dag_id).one_or_none()
    if not dag:
        raise NotFound(f"Dag with id: '{dag_id}' not found")
    try:
        patch_body = dag_schema.load(request.json, session=session)
    except ValidationError as err:
        raise BadRequest("Invalid Dag schema", detail=str(err.messages))
    if update_mask:
        patch_body_ = {}
        if len(update_mask) > 1:
            raise BadRequest(detail="Only `is_paused` field can be updated through the REST API")
        update_mask = update_mask[0]
        if update_mask != 'is_paused':
            raise BadRequest(detail="Only `is_paused` field can be updated through the REST API")
        patch_body_[update_mask] = patch_body[update_mask]
        patch_body = patch_body_
    setattr(dag, 'is_paused', patch_body['is_paused'])
    session.commit()
    return dag_schema.dump(dag)