Just got a new Macbook Pro and the above mentioned lines when forwarding mail did indeed appear. Compared it with my older Macbook Pro settings and got rid off any lines with the following preference changes:
Fonts & Colors:
leave the box "Color quoted text' unchecked
and open Leven One - Level three color choices
and choose WHITE
so (you might have to slide the color bar on the right all the way up, then a small center field of white will appear in the center of the circle, drag the point there)
Voila - perhaps the lines are still there, but now invisible!
Under Composing I do have the Increase quote level checked, but may not have any relevance.