Revert "grml-live: clean up MIRROR_DIRECTORY if provided."

This reverts commit c78f58b.

While deleting the bind-mount destination of mirror directories is fine
in theory, we end up deleting empty base directories as well, which
is unexpected and unwanted behavior.

See grml#104 for the related
discussion, this issue needs further attention yet before we can
include this in a new release of grml-live.

grml-live

@@ -180,10 +180,7 @@ umount_all() {
    fi
 
    umount "${CHROOT_OUTPUT}/grml-live/sources/" 2>/dev/null || /bin/true
-   if [ -n "${MIRROR_DIRECTORY}" ]; then
-     umount "${CHROOT_OUTPUT}/${MIRROR_DIRECTORY}"
-     rmdir -p "${CHROOT_OUTPUT}/${MIRROR_DIRECTORY}"
-   fi
+   [ -n "$MIRROR_DIRECTORY" ] && umount "${CHROOT_OUTPUT}/${MIRROR_DIRECTORY}"
 }
 # }}}