gettext: remove support for gettext-tools on target

This commit removes the BR2_PACKAGE_GETTEXT_TOOLS option, which could
be used to install gettext tools on the target. This is not needed,
because Buildroot is not designed to provide a full development
environment on the target, and gettext translation files should be
processed on the build machine, using the host gettext tools.

Remove this option will allow to optimize the build time of gettext on
the target, by only building the gettext runtime libraries.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Andrew Ruder <andrew.ruder@elecsyscorp.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>