**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
dagrun_1 = dag.create_dagrun(
            run_type=DagRunType.BACKFILL_JOB,
            state=State.RUNNING,
            start_date=DEFAULT_DATE,
            execution_date=DEFAULT_DATE,
        )
        session.merge(dagrun_1)
        task_instance_1 = TI(t_1, execution_date=DEFAULT_DATE, state=ti_state_begin)
        task_instance_1.job_id = 123
        session.merge(task_instance_1)
        session.commit()
        dag.clear(
            start_date=DEFAULT_DATE,
            end_date=DEFAULT_DATE + datetime.timedelta(days=1),
            session=session,
        )
        task_instances = (
            session.query(
                TI,
            )
            .filter(
                TI.dag_id == dag_id,
            )
            .all()
        )
        assert len(task_instances) == 1
        task_instance = task_instances[0]  
        assert task_instance.state == ti_state_end
        self._clean_up(dag_id)
    def test_next_dagrun_after_date_once(self):