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