**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 scheduling a dag scheduled for @once - should be scheduled the first time
        it is called, and not scheduled the second.
        """
        dag_id = "test_schedule_dag_once"
        dag = DAG(dag_id=dag_id)
        dag.schedule_interval = '@once'
        assert dag.normalized_schedule_interval is None
        dag.add_task(BaseOperator(task_id="faketastic", owner='Also fake', start_date=TEST_DATE))
        
        dag.sync_to_db()
        dag.create_dagrun(run_type=DagRunType.SCHEDULED, execution_date=TEST_DATE, state=State.SUCCESS)
        
        dag.sync_to_db()
        with create_session() as session:
            model = session.query(DagModel).get((dag.dag_id,))
        assert model.next_dagrun is None
        assert model.next_dagrun_create_after is None
        self._clean_up(dag_id)
    def test_fractional_seconds(self):