**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
"""
            Create a subdag.
            """
            dag_subdag = DAG(
                dag_id=f'{parent_dag_name}.{child_dag_name}',
                schedule_interval="@daily",
                default_args=args,
            )
            for i in range(2):
                DummyOperator(task_id=f'{child_dag_name}-task-{i + 1}', dag=dag_subdag)
            return dag_subdag
        with DAG(
            dag_id='test_subdag_operator',
            start_date=datetime.datetime(2019, 1, 1),
            max_active_runs=1,
            schedule_interval=timedelta(minutes=1),
        ) as dag:
            section_1 = SubDagOperator(
                task_id='section-1',
                subdag=subdag(dag.dag_id, 'section-1', {'start_date': dag.start_date}),
            )
        subdag = section_1.subdag
        
        subdag.parent_dag = dag
        subdag.is_subdag = True
        next_date = dag.next_dagrun_after_date(None)
        assert next_date == timezone.datetime(2019, 1, 1, 0, 0)
        next_subdag_date = subdag.next_dagrun_after_date(None)
        assert next_subdag_date is None, "SubDags should never have DagRuns created by the scheduler"
    def test_replace_outdated_access_control_actions(self):