sched/fair: Remove unused return of _nohz_idle_balance

The return of _nohz_idle_balance() is not used anymore so we can remove
it

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
Link: https://lkml.kernel.org/r/20210224133007.28644-3-vincent.guittot@linaro.org