**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
schedule_interval='*/10 * * * *',
            start_date=six_hours_ago_to_the_hour,
            catchup=False,
        )
        next_date = dag1.next_dagrun_after_date(None)
        
        assert next_date > half_an_hour_ago
        assert next_date < timezone.utcnow()
        dag2 = make_dag(
            dag_id='dag_without_catchup_hourly',
            schedule_interval='@hourly',
            start_date=six_hours_ago_to_the_hour,
            catchup=False,
        )
        next_date = dag2.next_dagrun_after_date(None)
        
        assert next_date > two_hours_ago
        
        assert next_date < timezone.utcnow()
        dag3 = make_dag(
            dag_id='dag_without_catchup_once',
            schedule_interval='@once',
            start_date=six_hours_ago_to_the_hour,
            catchup=False,
        )
        next_date = dag3.next_dagrun_after_date(None)
        
        assert next_date == six_hours_ago_to_the_hour
    @freeze_time(timezone.datetime(2020, 1, 5))
    def test_next_dagrun_after_date_timedelta_schedule_and_catchup_false(self):