ddclient: Fix incorrectly capitalized systemd unit key

This avoids the following warning:

Apr 19 10:53:48 xen systemd[1]: [/nix/store/...-unit-ddclient.service/ddclient.service:19] Unknown lvalue 'type' in section 'Service'

As `Type=simple` is the default in systemd, the assignment to the
service type can be simply dropped.