sched/fair: remove cpu.qos_leve from root cpu cgroup

jingdong inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I7R399



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/sys/fs/cgroup/cpu/cpu.qos_level is not needed, and easy
to cause misunderstanding.

Signed-off-by: zhangwei123171 <zhangwei123171@jd.com>
Reviewed-by: zhaoxiaoqiang11 <zhaoxiaoqiang11@jd.com>