Properly remove duplicate directories from systemd package

When removing duplicate directories from the systemd package, sort the
list of directories in reverse order so we properly delete nested
directories.

Running debdiff shows the following result:

Files in first .deb but not in second
-------------------------------------
drwxr-xr-x  root/root   /etc/udev/
drwxr-xr-x  root/root   /usr/lib/systemd/tests/
drwxr-xr-x  root/root   /usr/lib/systemd/tests/testdata/

Those empty directories clearly do not belong into systemd package.