### 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
ignore_all_deps = request.form.get('ignore_all_deps') == "true"
        ignore_task_deps = request.form.get('ignore_task_deps') == "true"
        ignore_ti_state = request.form.get('ignore_ti_state') == "true"
        executor = ExecutorLoader.get_default_executor()
        valid_celery_config = False
        valid_kubernetes_config = False
        try:
            from airflow.executors.celery_executor import CeleryExecutor
            valid_celery_config = isinstance(executor, CeleryExecutor)
        except ImportError:
            pass
        try:
            from airflow.executors.kubernetes_executor import KubernetesExecutor
            valid_kubernetes_config = isinstance(executor, KubernetesExecutor)
        except ImportError:
            pass
        if not valid_celery_config and not valid_kubernetes_config:
            flash("Only works with the Celery or Kubernetes executors, sorry", "error")
            return redirect(origin)
        ti = models.TaskInstance(task=task, execution_date=execution_date)
        ti.refresh_from_db()
        
        dep_context = DepContext(
            deps=RUNNING_DEPS,
            ignore_all_deps=ignore_all_deps,
            ignore_task_deps=ignore_task_deps,
            ignore_ti_state=ignore_ti_state,
        )
        failed_deps = list(ti.get_failed_dep_statuses(dep_context=dep_context))
        if failed_deps: