**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
"""
        We should never create dagruns for unpaused DAGs
        """
        dag = DAG(dag_id='test_dags', start_date=DEFAULT_DATE)
        DummyOperator(task_id='dummy', dag=dag, owner='airflow')
        session = settings.Session()
        orm_dag = DagModel(
            dag_id=dag.dag_id,
            has_task_concurrency_limits=False,
            next_dagrun=dag.start_date,
            next_dagrun_create_after=dag.following_schedule(DEFAULT_DATE),
            is_active=True,
        )
        session.add(orm_dag)
        session.flush()
        needed = DagModel.dags_needing_dagruns(session).all()
        assert needed == [orm_dag]
        orm_dag.is_paused = True
        session.flush()
        dag_models = DagModel.dags_needing_dagruns(session).all()
        assert dag_models == []
        session.rollback()
        session.close()
class TestQueries(unittest.TestCase):