[gtkmm/gtkmm-3-22] gtk-demo/example_headerbar: Remove an old comment

[Daniel Boles <dboles src gnome org>]

commit ef21e01266d74a04f5350b78b329f1a90bdf0a81
Author: Daniel Boles <dboles src gmail com>
Date:   Fri May 26 17:33:17 2017 +0100

    gtk-demo/example_headerbar: Remove an old comment
    
    Nearly 3 years later, I think we can safely assume these icons are
    available on the end-user’s system.

 demos/gtk-demo/example_headerbar.cc |    5 -----
 1 files changed, 0 insertions(+), 5 deletions(-)
---
diff --git a/demos/gtk-demo/example_headerbar.cc b/demos/gtk-demo/example_headerbar.cc
index 44022dc..09caad5 100644
--- a/demos/gtk-demo/example_headerbar.cc
+++ b/demos/gtk-demo/example_headerbar.cc
@@ -11,11 +11,6 @@
 
 #include "gtkmm.h"
 
-// 2014-09-19: The pan-[up,down,start,end]-symbolic icons are new.
-// See https://bugzilla.gnome.org/show_bug.cgi?id=729565
-// If they are not available in your selected icon theme, perhaps you can
-// use the go-[up,down,previous,next]-symbolic icons.
-
 class Example_HeaderBar : public Gtk::Window
 {
 public: