nscd: cleanup obsolete _FORTIFY_SOURCE setting

This was added in 233399bc just for warn_if_unused
warnings rather than anything substantial.

Now that we have a proper configure argument for F_S (--enable-fortify-source),
just drop this entirely, to avoid conflicting with e.g. detected --enable-fortify-source
finding F_S=3, then nscd's Makefile setting F_S=2, resulting in a build-failure
because of the redefinition.

Signed-off-by: Sam James <sam@gentoo.org>
Signed-off-by: Andreas K. Hüttel <dilfridge@gentoo.org>
Reviewed-by: Andreas K. Hüttel <dilfridge@gentoo.org>