libbpf: Clean up deprecated and legacy aliases

Remove three missed deprecated APIs that were aliased to new APIs:
bpf_object__unload, bpf_prog_attach_xattr and btf__load.

Also move legacy API libbpf_find_kernel_btf (aliased to
btf__load_vmlinux_btf) into libbpf_legacy.h.

Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Hao Luo <haoluo@google.com>
Link: https://lore.kernel.org/bpf/20220816001929.369487-4-andrii@kernel.org

src/btf.c

@@ -1225,8 +1225,6 @@ int btf__load_into_kernel(struct btf *btf)
 	return btf_load_into_kernel(btf, NULL, 0, 0);
 }
 
-int btf__load(struct btf *) __attribute__((alias("btf__load_into_kernel")));
-
 int btf__fd(const struct btf *btf)
 {
 	return btf->fd;