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/GCode/FanMover.cpp

@@ -27,8 +27,6 @@
 #include "../PrintConfig.hpp"
 #include "../Utils.hpp"
 #include "Print.hpp"
-#include "libslic3r/PrintConfig.hpp"
-#include "libslic3r/libslic3r.h"
 
 #include <boost/log/trivial.hpp>
 #include <string>