**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
assert dag < dag_subclass_diff_name
        
        assert dag_diff_name > dag
        
        assert hash(dag) == hash(dag)
        assert hash(dag_eq) == hash(dag)
        assert hash(dag_diff_name) != hash(dag)
        assert hash(dag_subclass) != hash(dag)
    def test_get_paused_dag_ids(self):
        dag_id = "test_get_paused_dag_ids"
        dag = DAG(dag_id, is_paused_upon_creation=True)
        dag.sync_to_db()
        assert DagModel.get_dagmodel(dag_id) is not None
        paused_dag_ids = DagModel.get_paused_dag_ids([dag_id])
        assert paused_dag_ids == {dag_id}
        with create_session() as session:
            session.query(DagModel).filter(DagModel.dag_id == dag_id).delete(synchronize_session=False)
    @parameterized.expand(
        [
            (None, None),
            ("@daily", "0 0 * * *"),
            ("@weekly", "0 0 * * 0"),
            ("@monthly", "0 0 1 * *"),
            ("@quarterly", "0 0 1 */3 *"),
            ("@yearly", "0 0 1 1 *"),
            ("@once", None),
            (datetime.timedelta(days=1), datetime.timedelta(days=1)),
        ]
    )
    def test_normalized_schedule_interval(self, schedule_interval, expected_n_schedule_interval):