selftests/bpf: Remove kmem subtest from cgroup_iter_memcg

selftests/bpf: Remove kmem subtest from cgroup_iter_memcg

When cgroup.memory=nokmem is set in the kernel command line, kmem
accounting is disabled. This causes the test_kmem subtest in
cgroup_iter_memcg to fail because it expects non-zero kmem values.

Remove the kmem subtest altogether since the remaining subtests
(shmem, file, pgfault) already provide sufficient coverage for
the cgroup iter memcg functionality.

Reviewed-by: JP Kobryn <jp.kobryn@linux.dev>
Signed-off-by: Hui Zhu <zhuhui@kylinos.cn>
Link: https://lore.kernel.org/r/35fa32a019361ec26265c8a789ee31e448d4dbda.1772505399.git.zhuhui@kylinos.cn
Signed-off-by: Alexei Starovoitov <ast@kernel.org>