light_apply_rgb maybe vestigial

This function in `lighting/lighting` seems to only be used in `model/modelrender`'s `model_render_insignias()`. That itself appears to do C++-side per-vertex lighting on insignia meshes and then submit them to draw as unlit geometry. I'm pretty sure we've got some drawing issues on the tracker for insignia and this really seems like a old crufty way to do things that's at odds with how we draw everything else. If that's all accurate, then we probably ought to do something about that eventually