libbpf: remove deprecated BTF APIs

Get rid of deprecated BTF-related APIs.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/r/20220627211527.2245459-6-andrii@kernel.org


Signed-off-by: Alexei Starovoitov <ast@kernel.org>