ddclient is unmaintained as of 2023-07-04 [1] and this service has been broken
for a while [2]. Remove it rather than shipping a broken service for an
unmaintained program that's unlikely to be fixed.
[1]: <https://github.com/ddclient/ddclient>
[2]: <https://issues.guix.gnu.org/52770>
This reverts commit 8490a8346b.
Signed-off-by: Ludovic Courtès <ludo@gnu.org>