selftests/bpf: remove bpf_util.h from BPF C progs

Though currently there is no problem including bpf_util.h in kernel
space BPF C programs, in next patch in this stack, I will reuse
libbpf_num_possible_cpus() in bpf_util.h thus include libbpf.h in it,
which will cause BPF C programs compile error. Therefore I will first
remove bpf_util.h from all test BPF programs.

This can also make it clear that bpf_util.h is a user-space utility
while bpf_helpers.h is a kernel space utility.

Signed-off-by: Hechao Li <hechaol@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>