**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_id = 'test_clear_set_dagrun_state_subdag'
        self._clean_up(dag_id)
        task_id = 't1'
        dag = DAG(dag_id, start_date=DEFAULT_DATE, max_active_runs=1)
        t_1 = DummyOperator(task_id=task_id, dag=dag)
        subdag = DAG(dag_id + '.test', start_date=DEFAULT_DATE, max_active_runs=1)
        SubDagOperator(task_id='test', subdag=subdag, dag=dag)
        t_2 = DummyOperator(task_id='task', dag=subdag)
        session = settings.Session()
        dagrun_1 = dag.create_dagrun(
            run_type=DagRunType.BACKFILL_JOB,
            state=State.FAILED,
            start_date=DEFAULT_DATE,
            execution_date=DEFAULT_DATE,
        )
        dagrun_2 = subdag.create_dagrun(
            run_type=DagRunType.BACKFILL_JOB,
            state=State.FAILED,
            start_date=DEFAULT_DATE,
            execution_date=DEFAULT_DATE,
        )
        session.merge(dagrun_1)
        session.merge(dagrun_2)
        task_instance_1 = TI(t_1, execution_date=DEFAULT_DATE, state=State.RUNNING)
        task_instance_2 = TI(t_2, execution_date=DEFAULT_DATE, state=State.RUNNING)