**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
outdated_permissions = {
            'role1': {permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT},
            'role2': {permissions.DEPRECATED_ACTION_CAN_DAG_READ, permissions.DEPRECATED_ACTION_CAN_DAG_EDIT},
        }
        updated_permissions = {
            'role1': {permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT},
            'role2': {permissions.ACTION_CAN_READ, permissions.ACTION_CAN_EDIT},
        }
        with pytest.warns(DeprecationWarning):
            dag = DAG(dag_id='dag_with_outdated_perms', access_control=outdated_permissions)
        assert dag.access_control == updated_permissions
        with pytest.warns(DeprecationWarning):
            dag.access_control = outdated_permissions
        assert dag.access_control == updated_permissions
class TestDagModel: