[babl] babl: remove outdated comment

[Øyvind "pippin" Kolås <ok src gnome org>]

commit 95c3d9ac5b808fb081d03c762bc59aa2610998aa
Author: Øyvind Kolås <pippin gimp org>
Date:   Wed Jun 20 12:55:55 2018 +0200

    babl: remove outdated comment

 babl/base/model-gray.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)
---
diff --git a/babl/base/model-gray.c b/babl/base/model-gray.c
index 5449a4f..5db405b 100644
--- a/babl/base/model-gray.c
+++ b/babl/base/model-gray.c
@@ -201,7 +201,7 @@ rgb_to_gray_nonlinear (Babl  *conversion,
       else
         alpha = 1.0;
 
-      luminance = red   * RGB_LUMINANCE_RED +    // XXX: should be taken from BablSpace
+      luminance = red   * RGB_LUMINANCE_RED +
                   green * RGB_LUMINANCE_GREEN +
                   blue  * RGB_LUMINANCE_BLUE;
       *(double *) dst[0] = babl_trc_from_linear (trc, luminance);