Hi Michal,
Based on your query we suspect that you want not to render the Group header (Marked in screen shot) inside Grid rows. By default collapse/expand functions are done by using those icon which is in the row. If we remove those Group header it leads to complete layout change in Grid. So, it is not feasible to remove those Group header lines inside the Grid rows.
Refer the screen shot.
If we misunderstood your query then please get back to us.
Regards,
Thavasianand S.