backend: remove default implementations of obsolete backend operations

src/backend/backend_common.h

@@ -54,14 +54,6 @@ solid_picture(struct x_connection *, bool argb, double a, double r, double g, do
 xcb_image_t *make_shadow(struct x_connection *c, const conv *kernel, double opacity,
                          int width, int height);
 
-/// The default implementation of `is_win_transparent`, it simply looks at win::mode. So
-/// this is not suitable for backends that alter the content of windows
-bool default_is_win_transparent(void *, win *, void *);
-
-/// The default implementation of `is_frame_transparent`, it uses win::frame_opacity. Same
-/// caveat as `default_is_win_transparent` applies.
-bool default_is_frame_transparent(void *, win *, void *);
-
 void *default_render_shadow(backend_t *backend_data, int width, int height,
                             struct backend_shadow_context *sctx, struct color color);