**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
local_tz = pendulum.timezone('Europe/Zurich')
        default_args = {'start_date': datetime.datetime(2018, 1, 1, tzinfo=local_tz)}
        dag = DAG('DAG', default_args=default_args)
        assert dag.timezone.name == local_tz.name
        dag = DAG('DAG', default_args=default_args)
        assert dag.timezone.name == local_tz.name
    def test_roots(self):
        """Verify if dag.roots returns the root tasks of a DAG."""
        with DAG("test_dag", start_date=DEFAULT_DATE) as dag:
            op1 = DummyOperator(task_id="t1")
            op2 = DummyOperator(task_id="t2")
            op3 = DummyOperator(task_id="t3")
            op4 = DummyOperator(task_id="t4")
            op5 = DummyOperator(task_id="t5")
            [op1, op2] >> op3 >> [op4, op5]
            assert set(dag.roots) == {op1, op2}
    def test_leaves(self):