The cpus2use.sh script complains if the mpstat command is not available,
and instead uses all available CPUs.  Unfortunately, this complaint
goes to stdout, where it confuses invokers who expect a single number.
This commit removes this error message in order to avoid this confusion.
The tendency of late has been to give rcutorture a full system, so this
should not cause issues.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>