build: clear 54 warnings - dead private fields (#15423)

build: clear 54 dead private fields

54 of the 161 -Wunused-private-field warnings, across 31 files. These are
the ones needing no judgment. Each member is declared once and appears
nowhere else in src/, counting the .mm and .c sources as well as .cpp and
.hpp, so nothing writes them and nothing reads them. Every removal is a
whole line, and no declaration shares a line with another member.

The remaining 107 are left alone. Those members are mentioned elsewhere,
usually assigned and never read, where the fix might be deleting the
member or might be restoring a read that went missing.

src/slic3r/GUI/GCodeViewer.hpp

@@ -72,7 +72,6 @@ public:
             float m_model_z_offset{ 0.5f };
             bool m_visible{ true };
             bool m_is_dark = false;
-            bool m_fixed_screen_size{ false };
             float m_scale_factor{ 1.0f };
 #if ENABLE_ACTUAL_SPEED_DEBUG
             ActualSpeedImguiWidget m_actual_speed_imgui_widget;