directfb/legacy: add that it was removed

As pointed by Yann.

Signed-off-by: Gustavo Zacarias <gustavo.zacarisa@free-electrons.com>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>