**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
with create_session() as session:
            session.add(DagModel(dag_id='dag_default_view_old', default_view=None))
            session.commit()
            orm_dag = session.query(DagModel).filter(DagModel.dag_id == 'dag_default_view_old').one()
        assert orm_dag.default_view is None
        assert orm_dag.get_default_view() == conf.get('webserver', 'dag_default_view').lower()
    def test_dag_is_deactivated_upon_dagfile_deletion(self):
        dag_id = 'old_existing_dag'
        dag_fileloc = "/usr/local/airflow/dags/non_existing_path.py"
        dag = DAG(
            dag_id,
            is_paused_upon_creation=True,
        )
        dag.fileloc = dag_fileloc
        session = settings.Session()
        with mock.patch.object(settings, "STORE_DAG_CODE", False):
            dag.sync_to_db(session=session)
        orm_dag = session.query(DagModel).filter(DagModel.dag_id == dag_id).one()
        assert orm_dag.is_active
        assert orm_dag.fileloc == dag_fileloc
        DagModel.deactivate_deleted_dags(list_py_file_paths(settings.DAGS_FOLDER))
        orm_dag = session.query(DagModel).filter(DagModel.dag_id == dag_id).one()
        assert not orm_dag.is_active
        
        session.execute(DagModel.__table__.delete().where(DagModel.dag_id == dag_id))
        session.close()
    def test_dag_naive_default_args_start_date_with_timezone(self):