gettext: remove libintl handling

Now, the gettext package always:

 * install everything to the staging directory
 * install the libraries to the target directory

Note that this commit removes BR2_PACKAGE_LIBINTL without updating the
packages, it is done in a separate commit to ease the review process.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[yann.morin.1998@free.fr: keep BR2_PACKAGE_LIBINTL as blind option]
Signed-off-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
Tested-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Acked-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
CC: Samuel Martin <s.martin49@gmail.com>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>