defaults/rc.conf: Remove /usr/lib32 from ldconfig32_paths

Commit 99132daf6f70cb0cc969c555d3612547fa3cf1db prepends /usr/lib32 to
the list of paths in ldconfig32_paths since it is a standard library
path in ld-elf32.so.1. Remove /usr/lib32 from the value in rc.conf so
that it is not listed twice.

Reviewed by: olce, kib
Sponsored by: University of Cambridge, Google, Inc.
Differential Revision: https://reviews.freebsd.org/D44752