math: Remove leftover noop conversion and comment from `Rgba::reflect…

…()`.

Now that the alpha component is `ZeroOne` and the other numbers involved
are `PositiveSign`, no NaN checks are present.

all-is-cubes-base/src/math/color.rs

@@ -425,9 +425,7 @@ impl Rgba {
     #[inline]
     #[doc(hidden)] // not sure if good public API
     pub fn reflect(self, illumination: Rgb) -> Rgb {
-        // TODO: do this math without any NaN checks or negative/amplified values.
-        // by introducing a dedicated RgbaReflectance type with constrained components?
-        self.to_rgb() * illumination * PositiveSign::from(self.alpha)
+        self.to_rgb() * illumination * self.alpha
     }
 }