**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
row[0] for row in session.query(DagModel.dag_id).all()
            }
            assert {
                ('dag-bulk-sync-0', 'test-dag'),
                ('dag-bulk-sync-0', 'test-dag2'),
                ('dag-bulk-sync-1', 'test-dag'),
                ('dag-bulk-sync-1', 'test-dag2'),
                ('dag-bulk-sync-2', 'test-dag'),
                ('dag-bulk-sync-2', 'test-dag2'),
                ('dag-bulk-sync-3', 'test-dag'),
                ('dag-bulk-sync-3', 'test-dag2'),
            } == set(session.query(DagTag.dag_id, DagTag.name).all())
        
        for dag in dags:
            dag.tags.remove("test-dag")
        with assert_queries_count(5):
            DAG.bulk_write_to_db(dags)
        with create_session() as session:
            assert {'dag-bulk-sync-0', 'dag-bulk-sync-1', 'dag-bulk-sync-2', 'dag-bulk-sync-3'} == {
                row[0] for row in session.query(DagModel.dag_id).all()
            }
            assert {
                ('dag-bulk-sync-0', 'test-dag2'),
                ('dag-bulk-sync-1', 'test-dag2'),
                ('dag-bulk-sync-2', 'test-dag2'),
                ('dag-bulk-sync-3', 'test-dag2'),
            } == set(session.query(DagTag.dag_id, DagTag.name).all())
            for row in session.query(DagModel.last_parsed_time).all():
                assert row[0] is not None
    def test_bulk_write_to_db_max_active_runs(self):