giflib: remove from the description the reference to libungif

Starting from version 5.0 giflib it's no longer compatible with
libungif, also the libungif is now deprecated so just remove the
reference to libungif.

Signed-off-by: Fabio Porcedda <fabio.porcedda@gmail.com>
Cc: Yann E. MORIN <yann.morin.1998@free.fr>
Cc: Baruch Siach <baruch@tkos.co.il>
Acked-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Reviewed-by: Baruch Siach <baruch@tkos.co.il>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>