window: remove window_set_transparent() declaration

The function is not implemented.

Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>

clients/window.h

@@ -253,8 +253,6 @@ window_move(struct window *window, struct input *input, uint32_t time);
 void
 window_get_allocation(struct window *window, struct rectangle *allocation);
 void
-window_set_transparent(struct window *window, int transparent);
-void
 window_schedule_redraw(struct window *window);
 void
 window_schedule_resize(struct window *window, int width, int height);