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

This reverts commit e5934962.

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/9958Reviewed-by: Stephan Bergmann <sbergman@redhat.com>
Tested-by: Stephan Bergmann <sbergman@redhat.com>

configure.ac

@@ -7880,11 +7880,7 @@ if test "$with_system_libxml" = "auto"; then
         with_system_libxml="$with_system_libs"
         ;;
     *)
-        if test "$_os" = "Darwin" -a $MAC_OS_X_VERSION_MAX_ALLOWED -lt 1060; then
-            with_system_libxml=no
-        else
-            with_system_libxml=yes
-        fi
+        with_system_libxml=yes
         ;;
     esac
 fi