[gtk+] Remove outdated comments

[William Hua <williamhua src gnome org>]

commit b2719c0383f68955e84b368d893caa43c4ec61e2
Author: Andreas Pokorny <andreas pokorny canonical com>
Date:   Tue Jul 19 10:46:06 2016 +0200

    Remove outdated comments
    
    https://bugzilla.gnome.org/show_bug.cgi?id=768138

 gdk/mir/gdkmirwindowimpl.c |    1 -
 1 files changed, 0 insertions(+), 1 deletions(-)
---
diff --git a/gdk/mir/gdkmirwindowimpl.c b/gdk/mir/gdkmirwindowimpl.c
index 46d92e2..fc86349 100644
--- a/gdk/mir/gdkmirwindowimpl.c
+++ b/gdk/mir/gdkmirwindowimpl.c
@@ -595,7 +595,6 @@ gdk_mir_window_impl_ref_cairo_surface (GdkWindow *window)
 
   ensure_surface (window);
 
-  /* Transient windows get rendered into a buffer and copied onto their parent */
   if (window->gl_paint_context)
     {
       cairo_surface = cairo_image_surface_create (pixel_format, window->width, window->height);