Cleanup: Remove unused Voronoi texture code.

Let's finally delete this code, after 4 years of being unused,
there really is no excuse anymore.

If we decide to extend the procedural textures in SVM, we can do this anytime in the future.

intern/cycles/kernel/shaders/node_voronoi_texture.osl

@@ -37,7 +37,7 @@ shader node_voronoi_texture(
 	float da[4];
 	point pa[4];
 
-	voronoi(p * Scale, "Distance Squared", 1.0, da, pa);
+	voronoi(p * Scale, 1.0, da, pa);
 
 	/* Colored output */
 	if (Coloring == "Intensity") {