LU-2800 autoconf: remove LIBCFS_CPUMASK_SIZE

LU-2800 autoconf: remove LIBCFS_CPUMASK_SIZE

The function cpumask_size has been in the linux kernel
since 2.6.28. Also HAVE_CPUMASK_SIZE is not even used
anywhere in the source tree. We can remove this obsolete
test.

Signed-off-by: James Simmons <uja.ornl@gmail.com>
Change-Id: Ie6090055105dcaa55fc031bf52e5a25cd7bcf321
Reviewed-on: http://review.whamcloud.com/7693
Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Minh Diep <minh.diep@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>