**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
if task_end_dates and next_run_date:
            min_task_end_date = min(task_end_dates)
            if next_run_date > min_task_end_date:
                return None
        return next_run_date
    def get_run_dates(self, start_date, end_date=None):
        """
        Returns a list of dates between the interval received as parameter using this
        dag's schedule interval. Returned dates can be used for execution dates.
        :param start_date: the start date of the interval
        :type start_date: datetime
        :param end_date: the end date of the interval, defaults to timezone.utcnow()
        :type end_date: datetime
        :return: a list of dates within the interval following the dag's schedule
        :rtype: list
        """
        run_dates = []
        using_start_date = start_date
        using_end_date = end_date
        
        using_start_date = using_start_date or min([t.start_date for t in self.tasks])
        using_end_date = using_end_date or timezone.utcnow()
        
        
        next_run_date = self.normalize_schedule(using_start_date) if not self.is_subdag else using_start_date
        while next_run_date and next_run_date <= using_end_date:
            run_dates.append(next_run_date)
            next_run_date = self.following_schedule(next_run_date)
        return run_dates
    def normalize_schedule(self, dttm):