Perhaps…
togo = []
grp.entities.grep(Sketchup::Edge).each{|e|
faces= e.faces
togo << e if faces.length == 2 && faces[0].normal.parallel?(faces[1].normal)
togo << e if faces.length == 0
}
grp.entities.erase_entities( togo )
To delete edges dividing coplanar faces and unfaced edges…