cgroup:namespace: Remove unused cgroup_namespaces_init()

cgroup_namspace_init() just return 0. Therefore, there is no need to
call it during start_kernel. Just remove it.

Fixes: a79a908f ("cgroup: introduce cgroup namespaces")
Signed-off-by: Lu Jialin <lujialin4@huawei.com>
Signed-off-by: Tejun Heo <tj@kernel.org>