ldconfig: remove i386 aout invocation

aout support in ldconfig hasn't been required since FreeBSD 2.x.

Anyone still using FreeBSD 2 shared libraries can use a FreeBSD 2
ldconfig to generate aout ldconfig hints.

Reviewed by: dim, kib
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D24883