ddclient: T1789: fix RFC2136 generated config

Commit 967067970494c1800f ("ddclient: T1030: adjust to latest syntax")
was under the impression that ddclient 3.9.0 now handles every config
item with a comma in the end. This is unfortunately not true on RFC2136
dynamic DNS entries.

Remove commas on config template.

(cherry picked from commit cc2ea329b1bb2ac23ffcc64892e831e7978023e2)