cpumask: remove deprecated functions.

Using these functions with offstack cpus is unsafe.  They use all NR_CPUS
bits, unstead of nr_cpumask_bits.

In particular, lustre (in staging) used cpus_ and that caused a bug.

Reported-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>