selftests/bpf: test_progs: test__printf -> printf

Now that test__printf is a simple wraper around printf, let's drop it
(and test__vprintf as well).

Cc: Andrii Nakryiko <andriin@fb.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Signed-off-by: Stanislav Fomichev <sdf@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>