**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
)
        assert {
            (dag_id, False),
            (subdag_id, False),
        } == set(unpaused_dags)
        DagModel.get_dagmodel(dag.dag_id).set_is_paused(is_paused=True, including_subdags=False)
        paused_dags = (
            session.query(DagModel.dag_id, DagModel.is_paused)
            .filter(
                DagModel.dag_id.in_([subdag_id, dag_id]),
            )
            .all()
        )
        assert {
            (dag_id, True),
            (subdag_id, False),
        } == set(paused_dags)
        DagModel.get_dagmodel(dag.dag_id).set_is_paused(is_paused=True)
        paused_dags = (
            session.query(DagModel.dag_id, DagModel.is_paused)
            .filter(
                DagModel.dag_id.in_([subdag_id, dag_id]),
            )
            .all()
        )
        assert {
            (dag_id, True),
            (subdag_id, True),
        } == set(paused_dags)
    def test_existing_dag_is_paused_upon_creation(self):