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

tools/testing/selftests/bpf/prog_tests/btf_dedup_split.c

@@ -364,7 +364,7 @@ static void test_split_dup_struct_in_cu()
 			"\t'f2' type_id=1 bits_offset=32");
 
 	/* ..dedup them... */
-	err = btf__dedup(btf1, NULL, NULL);
+	err = btf__dedup(btf1, NULL);
 	if (!ASSERT_OK(err, "btf_dedup"))
 		goto cleanup;
 
@@ -405,7 +405,7 @@ static void test_split_dup_struct_in_cu()
 			"\t'f1' type_id=4 bits_offset=0\n"
 			"\t'f2' type_id=4 bits_offset=32");
 
-	err = btf__dedup(btf2, NULL, NULL);
+	err = btf__dedup(btf2, NULL);
 	if (!ASSERT_OK(err, "btf_dedup"))
 		goto cleanup;