selftests: bpf: Remove unused bpf_map_def_legacy struct

samples/bpf no longer use bpf_map_def_legacy and instead use the
libbpf's bpf_map_def or new BTF-defined MAP format. This commit removes
unused bpf_map_def_legacy struct from selftests/bpf/bpf_legacy.h.

Signed-off-by: Daniel T. Lee <danieltimlee@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Link: https://lore.kernel.org/bpf/20200707184855.30968-5-danieltimlee@gmail.com