shorthand: Don't require pack/unpack functions

When using G_TYPE_NONE as the shorthand's type, the property is not
gettable/settable using regular APIs, so it essentially doesn't exist.
As it should be.

gtk/gtkcssshorthandpropertyimpl.c

@@ -616,9 +616,6 @@ _gtk_css_shorthand_property_register (const char                        *name,
 {
   GtkStyleProperty *node;
 
-  g_return_if_fail (pack_func != NULL);
-  g_return_if_fail (unpack_func != NULL);
-
   node = g_object_new (GTK_TYPE_CSS_SHORTHAND_PROPERTY,
                        "name", name,
                        "value-type", value_type,