selftests/bpf: Stop using relaxed_core_relocs which has no effect

relaxed_core_relocs option hasn't had any effect for a while now, stop
specifying it. Next patch marks it as deprecated.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210916015836.1248906-3-andrii@kernel.org