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/GLToolbar.hpp

@@ -327,7 +327,6 @@ private:
     GLTexture m_icons_texture;
     bool m_icons_texture_dirty;
     mutable GLTexture m_images_texture;
-    mutable bool m_images_texture_dirty;
     BackgroundTexture m_background_texture;
     GLTexture m_arrow_texture;
     Layout m_layout;