**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
"""
        Tests following_schedule a dag with a relativedelta schedule_interval
        """
        dag_id = "test_schedule_dag_relativedelta"
        delta = relativedelta(hours=+1)
        dag = DAG(dag_id=dag_id, schedule_interval=delta)
        dag.add_task(BaseOperator(task_id="faketastic", owner='Also fake', start_date=TEST_DATE))
        _next = dag.following_schedule(TEST_DATE)
        assert _next.isoformat() == "2015-01-02T01:00:00+00:00"
        _next = dag.following_schedule(_next)
        assert _next.isoformat() == "2015-01-02T02:00:00+00:00"
    def test_dagtag_repr(self):
        clear_db_dags()
        dag = DAG('dag-test-dagtag', start_date=DEFAULT_DATE, tags=['tag-1', 'tag-2'])
        dag.sync_to_db()
        with create_session() as session:
            assert {'tag-1', 'tag-2'} == {
                repr(t) for t in session.query(DagTag).filter(DagTag.dag_id == 'dag-test-dagtag').all()
            }
    def test_bulk_write_to_db(self):