Revert "use bundled (non-system) versions of libxml2 and libxslt for OS X <10.6"

This reverts commit e5934962bd55970c54b399351559e90d02b75b6b.

It never worked on 10.5 (but worked for >10.5) actually due to xsltproc
and make's "Circular dependency dropped" issues.

Change-Id: I8770d99a86deb5276bf28dc38da8a5165426e1ef
Reviewed-on: https://gerrit.libreoffice.org/9958
Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>