lib/cpumask: drop always-true preprocessor guard

Since lib/cpumask.o is only built for CONFIG_SMP=y, NR_CPUS will always
be greater than 1 at compile time.  This makes checking for that
condition unnecesarry, so it can be dropped.

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Yury Norov <yury.norov@gmail.com>