shell-util: Remove shell_util_icon_from_string

GJS doesn't need to be able to represent interfaces for you to be able to
access an interface method, so Gio.icon_new_for_string works fine.

https://bugzilla.gnome.org/show_bug.cgi?id=661231

src/shell-util.h

@@ -11,8 +11,6 @@ G_BEGIN_DECLS
 
 char    *shell_util_get_label_for_uri          (const char       *text_uri);
 GIcon   *shell_util_get_icon_for_uri           (const char       *text_uri);
-GIcon   *shell_util_icon_from_string           (const char       *string,
-                                                GError          **error);
 
 void     shell_util_set_hidden_from_pick       (ClutterActor     *actor,
                                                 gboolean          hidden);