**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
"""A tag name per dag, to allow quick filtering in the DAG view."""
    __tablename__ = "dag_tag"
    name = Column(String(100), primary_key=True)
    dag_id = Column(String(ID_LEN), ForeignKey('dag.dag_id'), primary_key=True)
    def __repr__(self):
        return self.name
class DagModel(Base):
    """Table containing DAG properties"""
    __tablename__ = "dag"
    """
    These items are stored in the database for state related information
    """
    dag_id = Column(String(ID_LEN), primary_key=True)
    root_dag_id = Column(String(ID_LEN))
    
    
    is_paused_at_creation = conf.getboolean('core', 'dags_are_paused_at_creation')
    is_paused = Column(Boolean, default=is_paused_at_creation)
    
    is_subdag = Column(Boolean, default=False)
    
    is_active = Column(Boolean, default=False)
    
    last_parsed_time = Column(UtcDateTime)
    
    last_pickled = Column(UtcDateTime)
    
    
    last_expired = Column(UtcDateTime)