**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
"""
        Test to check that a DAG with catchup = False only schedules beginning now, not back to the start date
        """
        def make_dag(dag_id, schedule_interval, start_date, catchup):
            default_args = {
                'owner': 'airflow',
                'depends_on_past': False,
            }
            dag = DAG(
                dag_id,
                schedule_interval=schedule_interval,
                start_date=start_date,
                catchup=catchup,
                default_args=default_args,
            )
            op1 = DummyOperator(task_id='t1', dag=dag)
            op2 = DummyOperator(task_id='t2', dag=dag)
            op3 = DummyOperator(task_id='t3', dag=dag)
            op1 >> op2 >> op3
            return dag
        now = timezone.utcnow()
        six_hours_ago_to_the_hour = (now - datetime.timedelta(hours=6)).replace(
            minute=0, second=0, microsecond=0
        )
        half_an_hour_ago = now - datetime.timedelta(minutes=30)
        two_hours_ago = now - datetime.timedelta(hours=2)
        dag1 = make_dag(
            dag_id='dag_without_catchup_ten_minute',