**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
airflow/models/dag.py
do_it = True
        if count == 0:
            return 0
        if confirm_prompt:
            ti_list = "\n".join([str(t) for t in tis])
            question = (
                "You are about to delete these {count} tasks:\n{ti_list}\n\nAre you sure? (yes/no): "
            ).format(count=count, ti_list=ti_list)
            do_it = utils.helpers.ask_yesno(question)
        if do_it:
            clear_task_instances(
                tis,
                session,
                dag=self,
                activate_dag_runs=False,  
            )
            self.set_dag_runs_state(
                session=session,
                start_date=start_date,
                end_date=end_date,
                state=dag_run_state,
                dag_ids=dag_ids,
            )
        else:
            count = 0
            print("Cancelled, nothing was cleared.")
        session.commit()
        return count
    @classmethod
    def clear_dags(