**Apache Airflow version**: 2.0.0

**Environment**:

- **Cloud provider or hardware configuration**: Docker container
- **OS** (e.g. from /etc/os-release): Debian Stretch

**What happened**:

When working on some custom DAG override logic, I noticed that invoking `DAG.partial_subset` does not properly update the corresponding `_task_group.used_group_ids` on the returned subset DAG, such that adding back a task which was excluded during the `partial_subset` operation fails.

**What you expected to happen**:

Tasks that had already been added to the original DAG can be added again to the subset DAG returned by `DAG.partial_subset`

**How to reproduce it**:
Create any DAG with a single task called, e.g. `my-task`, then invoke `dag.partial_subset(['not-my-task'], False, False)`

Note that the returned subset DAG's `_task_group.used_group_ids` still contains `my-task` even though it was not included in the subset DAG itself

**Anything else we need to know**:

I was able to work around this by adding logic to update the new partial subset DAG's `_task_group.used_group_ids` manually, but this should really be done as part of the `DAG.partial_subset` logic
tests/models/test_dag.py
pass
        args = {'owner': 'airflow', 'start_date': DEFAULT_DATE}
        dag = DAG(test_dag_id, default_args=args)
        dag_eq = DAG(test_dag_id, default_args=args)
        dag_diff_load_time = DAG(test_dag_id, default_args=args)
        dag_diff_name = DAG(test_dag_id + '_neq', default_args=args)
        dag_subclass = DAGsubclass(test_dag_id, default_args=args)
        dag_subclass_diff_name = DAGsubclass(test_dag_id + '2', default_args=args)
        for dag_ in [dag_eq, dag_diff_name, dag_subclass, dag_subclass_diff_name]:
            dag_.last_loaded = dag.last_loaded
        
        assert dag == dag  
        
        assert dag_eq == dag
        assert dag_diff_name != dag
        assert dag_diff_load_time != dag
        
        assert dag_subclass != dag
        
        dump = pickle.dumps(dag)
        assert pickle.loads(dump) == dag
        
        assert dag < dag_diff_name
        assert dag > dag_diff_load_time