### 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
running_task_instance_query_result = session.query(
            TaskInstance.dag_id.label('dag_id'), TaskInstance.state.label('state')
        ).join(
            running_dag_run_query_result,
            and_(
                running_dag_run_query_result.c.dag_id == TaskInstance.dag_id,
                running_dag_run_query_result.c.execution_date == TaskInstance.execution_date,
            ),
        )
        if conf.getboolean('webserver', 'SHOW_RECENT_STATS_FOR_COMPLETED_RUNS', fallback=True):
            last_dag_run = (
                session.query(DagRun.dag_id, sqla.func.max(DagRun.execution_date).label('execution_date'))
                .join(DagModel, DagModel.dag_id == DagRun.dag_id)
                .filter(DagRun.state != State.RUNNING, DagModel.is_active)
                .group_by(DagRun.dag_id)
            )
            last_dag_run = last_dag_run.filter(DagRun.dag_id.in_(filter_dag_ids))
            last_dag_run = last_dag_run.subquery('last_dag_run')
            
            
            last_task_instance_query_result = session.query(
                TaskInstance.dag_id.label('dag_id'), TaskInstance.state.label('state')
            ).join(
                last_dag_run,
                and_(
                    last_dag_run.c.dag_id == TaskInstance.dag_id,
                    last_dag_run.c.execution_date == TaskInstance.execution_date,
                ),
            )
            final_task_instance_query_result = union_all(