You need at least 3 cps to create an arc. So, if you delete the bottom one, it'll be a line. You could play around with the bias values, but I would advise:
Select part of the arc that you want (highlight the line and not just the cp).
Press the Y key to make a new cp in the center of the line.
Now you can delte the bottom cp.
But why didn't you continue the spline down the cheeks to draw half of the face? And on that note, why haven't you go your roto lined up with your spline. Then you can just trace the line exactly with the spline.