cgroup.c: remove redundant check for mixable cgroup in cgroup_migrate_vet_dst

We have:
int cgroup_migrate_vet_dst(struct cgroup *dst_cgrp)
{
...
	/* mixables don't care */
	if (cgroup_is_mixable(dst_cgrp))
		return 0;

	/*
	 * If @dst_cgrp is already or can become a thread root or is
	 * threaded, it doesn't matter.
	 */
	if (cgroup_can_be_thread_root(dst_cgrp) || cgroup_is_threaded(dst_cgrp))
		return 0;
...
}

but in fact the entry of cgroup_can_be_thread_root() covers case that
checking cgroup_is_mixable() as following:
static bool cgroup_can_be_thread_root(struct cgroup *cgrp)
{
        /* mixables don't care */
        if (cgroup_is_mixable(cgrp))
                return true;
...
}

so explicitly checking in cgroup_migrate_vet_dst is unnecessary.

Signed-off-by: Lin Feng <linf@wangsu.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>