Fix of Can't convert polyline with more than two points to a line (#6933)

Sometimes Clipper produces a polyline with more than 2 points when
clipping a line with a polygon or a set of polygons. We hope the intermediate
points are collinear with the line, so we may just ignore them.

src/libslic3r/Polyline.hpp

@@ -62,7 +62,6 @@ public:
   
     const Point& last_point() const override { return this->points.back(); }
     const Point& leftmost_point() const;
-    Line line() const;
     Lines lines() const override;
 
     void clip_end(double distance);