sched/idle: Remove play_idle()

play_idle() is no longer in use, so delete it.

Signed-off-by: Feng Lee <379943137@qq.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/all/tencent_C3E0BD9B812C27A30FC49F1EA6A4B1352707@qq.com