ldd: Retire aout support

Userland aout support has not been required since FreeBSD 2.x.
If someone needs to use FreeBSD 2 shared libraries they will be best
served by using a FreeBSD 2 ldd, perhaps as part of a jail with a full
FreeBSD 2.x install.

Relnotes: Yes
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27478