nlist: retire long-obsolete aout support

[brooks]

lib/libc/gen/nlist.c

54

I'd be tempted to do an additional commit nuking _NLIST_DO_ELF as almost certainly pointless abstraction.

[emaste]

lib/libc/gen/nlist.c

54

Yep, I wanted to keep this separate.

[imp]

People (maybe just bde) were still builing aout kernels through the 5.x time frame, even though we'd cut over to ELF in 3.2... So yea, this is fine.
Do you need _NLIST_DO_ELF anymore? can't you unifdef things?

[emaste]

In D24229#533104, @imp wrote:

Do you need _NLIST_DO_ELF anymore? can't you unifdef things?

@brooks also suggested doing away with this; I think that's a good idea and will do in a followup commit.