selftests/bpf: Remove recently reintroduced legacy btf__dedup() use

We've added one extra patch that added back the use of legacy
btf__dedup() variant. Clean that up.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20211201232824.3166325-5-andrii@kernel.org