bpf: get rid of cgroup helper related ifdefs

As recently discussed during the task_under_cgroup_hierarchy() addition,
we should get rid of the ifdefs surrounding the bpf_skb_under_cgroup()
helper. If related functionality is not built-in, the helper cannot be
used anyway, which is also in line with what we do for all other helpers.

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>