**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
dag = DAG('dag', start_date=DEFAULT_DATE, default_args={'owner': 'owner1'})
        
        
        
        with dag:
            op1 = DummyOperator(task_id='A')
            op2 = DummyOperator(task_id='B')
            op3 = DummyOperator(task_id='C')
            op4 = DummyOperator(task_id='D')
            op1.set_upstream([op2, op3])
            op3.set_upstream(op4)
        topological_list = dag.topological_sort()
        logging.info(topological_list)
        tasks = [op2, op3, op4]
        assert topological_list[0] in tasks
        tasks.remove(topological_list[0])
        assert topological_list[1] in tasks
        tasks.remove(topological_list[1])
        assert topological_list[2] in tasks
        tasks.remove(topological_list[2])
        assert topological_list[3] == op1
    def test_dag_topological_sort2(self):