sched/fair: Remove duplicate #include

./kernel/sched/fair.c: linux/sched/cond_resched.h is included more than once.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Link: https://lore.kernel.org/r/20231018062759.44375-1-jiapeng.chong@linux.alibaba.com

Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=6907