**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 that DagModel.next_dagrun_create_after is set to NULL when the dag cannot be created due to max
        active runs being hit.
        """
        dag = DAG(dag_id='test_scheduler_verify_max_active_runs', start_date=DEFAULT_DATE)
        dag.max_active_runs = 1
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        session = settings.Session()
        dag.clear()
        DAG.bulk_write_to_db([dag], session)
        model = session.query(DagModel).get((dag.dag_id,))
        period_end = dag.following_schedule(DEFAULT_DATE)
        assert model.next_dagrun == DEFAULT_DATE
        assert model.next_dagrun_create_after == period_end
        dr = dag.create_dagrun(
            state=State.RUNNING,
            execution_date=model.next_dagrun,
            run_type=DagRunType.SCHEDULED,
            session=session,
        )
        assert dr is not None
        DAG.bulk_write_to_db([dag])
        model = session.query(DagModel).get((dag.dag_id,))
        assert model.next_dagrun == period_end
        
        assert model.next_dagrun_create_after is None
    def test_sync_to_db(self):