torture: Remove no-mpstat error message

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>