selftests/bpf: remove perf buffer helpers

libbpf's perf_buffer API supersedes trace_helper.h's helpers.
Remove those helpers after all existing users were already moved to
perf_buffer API.

Signed-off-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>