libungif: remove deprecated

It's been deprecated for a year now, so remove it.
While at it also remove the BR2_DEPRECATED_SINCE_2015_02 since it's now
unused.
And change the efl gif text to refer to giflib rather than libungif
which is what's used since it was deprecated.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>