Remove Duplicate Includes and the Dead GCodeSender Sources (#16249)

46 files include the same header twice at file scope, outside any #if,
66 times in all:
Model.cpp included Model.hpp twice, Utils.hpp <algorithm> and
<string_view> twice, seven GUI headers <wx/dataview.h> and
<wx/artprov.h> twice. The second include of each is removed.

GCodeSender.cpp and GCodeSender.hpp have been commented out of
libslic3r/CMakeLists.txt since 2022 and their only two includes are
commented out as well. Both files go, with the commented lines, and
the CMake entry for SLA/SupportTreeIGL.cpp, a file that no longer
exists.

src/libslic3r/CutSurface.cpp

@@ -2589,7 +2589,6 @@ void priv::create_face_types(FaceTypeMap           &map,
 }
 
 #include <CGAL/Polygon_mesh_processing/clip.h>
-#include <CGAL/Polygon_mesh_processing/corefinement.h>
 bool priv::clip_cut(SurfacePatch &cut, CutMesh clipper)
 {
     CutMesh& tm = cut.mesh;