sched/fair: find_idlest_group(): Remove unused sd_flag parameter

The last use of that parameter was removed by commit

  57abff06 ("sched/fair: Rework find_idlest_group()")

Get rid of the parameter.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Dietmar Eggemann <dietmar.eggemann@arm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lkml.kernel.org/r/20200415210512.805-2-valentin.schneider@arm.com