cgroup/cpuset: Remove cpuset_do_slab_mem_spread()

cgroup/cpuset: Remove cpuset_do_slab_mem_spread()

The SLAB allocator has been removed sine 6.8-rc1 [1], so there is no user
with SLAB_MEM_SPREAD and cpuset_do_slab_mem_spread(). Then SLAB_MEM_SPREAD
is marked as unused by [2]. Here we can remove
cpuset_do_slab_mem_spread(). For more details, please check [3].

[1] https://lore.kernel.org/linux-mm/20231120-slab-remove-slab-v2-0-9c9c70177183@suse.cz/
[2] https://lore.kernel.org/linux-kernel/20240223-slab-cleanup-flags-v2-0-02f1753e8303@suse.cz/T/
[3] https://lore.kernel.org/lkml/32bc1403-49da-445a-8c00-9686a3b0d6a3@redhat.com/T/#mf14b838c5e0e77f4756d436bac3d8c0447ea4350

Signed-off-by: Xiongwei Song <xiongwei.song@windriver.com>
Reviewed-by: Waiman Long <longman@redhat.com>
Signed-off-by: Tejun Heo <tj@kernel.org>