ldconfig: Retire a.out support

aout support in ldconfig hasn't 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 ldconfig as well.

In aa5e1b42e6e3 we removed the ldconfig a.out invocation from rc.d but
left the support in ldconfig itself. Remove it now.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27481