toolchain: drop BR2_NEEDS_GETTEXT{,_IF_LOCALE}

Now that all packages have been migrated to the new gettext logic, we
can remove the BR2_NEEDS_GETTEXT and BR2_NEEDS_GETTEXT_IF_LOCALE
variables.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>