**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
"""
        Make sure DST transitions are properly observed
        """
        local_tz = pendulum.timezone('Europe/Zurich')
        start = local_tz.convert(datetime.datetime(2018, 10, 28, 2, 55), dst_rule=pendulum.PRE_TRANSITION)
        assert start.isoformat() == "2018-10-28T02:55:00+02:00", "Pre-condition: start date is in DST"
        utc = timezone.convert_to_utc(start)
        dag = DAG('tz_dag', start_date=start, schedule_interval='*/5 * * * *')
        _next = dag.following_schedule(utc)
        next_local = local_tz.convert(_next)
        assert _next.isoformat() == "2018-10-28T01:00:00+00:00"
        assert next_local.isoformat() == "2018-10-28T02:00:00+01:00"
        prev = dag.previous_schedule(utc)
        prev_local = local_tz.convert(prev)
        assert prev_local.isoformat() == "2018-10-28T02:50:00+02:00"
        prev = dag.previous_schedule(_next)
        prev_local = local_tz.convert(prev)
        assert prev_local.isoformat() == "2018-10-28T02:55:00+02:00"
        assert prev == utc
    def test_following_previous_schedule_daily_dag_cest_to_cet(self):