sched/numa: remove unused nr_running field

nr_running in struct numa_stats is not used anywhere in the code.

Remove it.

Cc: Rik van Riel <riel@surriel.com>
Cc: Ingo Molnar <mingo@redhat.com>
Reviewed-by: Srikar Dronamraju <srikar@linux.vnet.ibm.com>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: http://lkml.kernel.org/r/1535548752-4434-3-git-send-email-vincent.guittot@linaro.org