treednd: Remove (out) annotation for GtkSelectionData arg

treednd: Remove (out) annotation for GtkSelectionData arg

gtk_tree_drag_source_drag_data_get's GtkSelectionData argument should not be
marked as (out) because:

a) GtkSelectionData is semi-private (it's declared in gtkselectionprivate.h),
and thus gobject-introspection has no knowledge of its fields or its size.
There is thus no way for language bindings to allocate GtkSelectionData.

b) Even if it was possible for language bindings to allocate GtkSelectionData,
a zeroed-out instance thus created would not be usable with
gtk_tree_drag_source_drag_data_get.  As far as I can tell, you need to
initialize its "target" member to the GdkAtom of "GTK_TREE_MODEL_ROW".
Language bindings have no way of knowing this, of course.

https://bugzilla.gnome.org/show_bug.cgi?id=692844