The Remove Wall Point command will remove a point from a room outline. The two walls connected at that point will be joined into one wall.
To remove a wall point, go to
Ribbon: DM HVAC->Rooms and Walls-> 
Remove Wall Point
Pulldown Menu: DM HVAC->Building Definition->Walls->Remove Wall Point
You will be prompted to specify the wall point to be removed.
Select wall near point to remove:
The point will be removed.