sched/fair: Remove useless check in select_idle_core()

The function select_idle_core() only gets called when has_idle_cores
is true which can be possible only when sched_smt_present is enabled.

This change also aligns select_idle_core() with select_idle_smt() in
the way that the caller do the check if necessary.

Signed-off-by: Abel Wu <wuyun.abel@bytedance.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Mel Gorman <mgorman@techsingularity.net>
Link: https://lore.kernel.org/r/20220907112000.1854-4-wuyun.abel@bytedance.com