**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
return list(self.task_dict.values())
    @tasks.setter
    def tasks(self, val):
        raise AttributeError('DAG.tasks can not be modified. Use dag.add_task() instead.')
    @property
    def task_ids(self) -> List[str]:
        return list(self.task_dict.keys())
    @property
    def task_group(self) -> "TaskGroup":
        return self._task_group
    @property
    def filepath(self) -> str:
        """File location of where the dag object is instantiated"""
        fn = self.full_filepath.replace(settings.DAGS_FOLDER + '/', '')
        fn = fn.replace(os.path.dirname(__file__) + '/', '')
        return fn
    @property
    def folder(self) -> str:
        """Folder location of where the DAG object is instantiated."""
        return os.path.dirname(self.full_filepath)
    @property
    def owner(self) -> str:
        """
        Return list of all owners found in DAG tasks.
        :return: Comma separated list of owners in DAG tasks
        :rtype: str
        """
        return ", ".join({t.owner for t in self.tasks})
    @property
    def allow_future_exec_dates(self) -> bool: