bpftool: Remove unused struct btf_attach_point

After commit 2828d0d7

 ("bpftool: Switch to libbpf's hashmap for
programs/maps in BTF listing"), struct btf_attach_point is not used
anymore and can be removed as well.

Signed-off-by: Yuan Can <yuancan@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220928090440.79637-2-yuancan@huawei.com