tick/sched : Remove redundant cpu_online() check

can_stop_idle_tick() checks cpu_online() twice. The first check leaves the
function when the CPU is not online, so the second check it
redundant. Remove it.

Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: fweisbec@gmail.com
Link: https://lkml.kernel.org/r/1539099815-2943-1-git-send-email-penghao122@sina.com.cn