selftests/bpf: get trusted cgrp from bpf_iter__cgroup directly

Commit f49843af

 (selftests/bpf: Add tests for css_task iter combining
with cgroup iter) added a test which demonstrates how css_task iter can be
combined with cgroup iter. That test used bpf_cgroup_from_id() to convert
bpf_iter__cgroup->cgroup to a trusted ptr which is pointless now, since
with the previous fix, we can get a trusted cgroup directly from
bpf_iter__cgroup.

Signed-off-by: Chuyi Zhou <zhouchuyi@bytedance.com>
Acked-by: Yonghong Song <yonghong.song@linux.dev>
Link: https://lore.kernel.org/r/20231107132204.912120-3-zhouchuyi@bytedance.com
Signed-off-by: Martin KaFai Lau <martin.lau@kernel.org>