sched/fair: Remove unused cfs_rq.idle_nr_running

cfs_rq.idle_nr_running field is not used anywhere so we can remove the
useless associated computation. Last user went in commit 5e963f2b
("sched/fair: Commit to EEVDF").

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Link: https://lore.kernel.org/r/20241202174606.4074512-9-vincent.guittot@linaro.org