Don't remove surface mode both cutting mesh areas.

The polylines are recomputed by the stitching, so they should be cleared.
However, the polygons should not be cleared for surface mode BOTH.
They are already cleared for surface mode SURFACE, 35 lines above at line 150.

Fixes CURA-9037

src/multiVolumes.cpp

@@ -184,7 +184,6 @@ void MultiVolumes::carveCuttingMeshes(std::vector<Slicer*>& volumes, const std::
             if (cutting_mesh.settings.get<ESurfaceMode>("magic_mesh_surface_mode") != ESurfaceMode::NORMAL)
             {
                 cutting_mesh_polylines.clear();
-                cutting_mesh_polygons.clear();
                 PolylineStitcher<Polygons, Polygon, Point>::stitch(new_polylines, cutting_mesh_polylines, cutting_mesh_polygons, surface_line_width);
             }
         }