cgroup/misc: Remove unused misc_cg_res_total_usage

misc_cg_res_total_usage() was added in 2021 by
commit a72232ea ("cgroup: Add misc cgroup controller")

but has remained unused.

Remove it.

Signed-off-by: Dr. David Alan Gilbert <linux@treblig.org>
Acked-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>