cgroup/cpuset: Avoid clearing CS_SCHED_LOAD_BALANCE twice

In cpuset_css_online(), CS_SCHED_LOAD_BALANCE will be cleared twice,
the former one in the is_in_v2_mode() case could be removed because
is_in_v2_mode() can be true for cgroup v1 if the "cpuset_v2_mode"
mount option is specified, that balance flag change isn't appropriate
for this particular case.

Signed-off-by: Xiu Jianfeng <xiujianfeng@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>