ddns-scripts: remove old and unmaintainded uci defaults scripts

ddns-scripts: remove old and unmaintainded uci defaults scripts

From my point of view there are several reasons why this uci default
script should be deleted.

- This script is no longer maintained and there was no significant
change since the old stable release openwrt-18.06.

- The script is installed with every additional package. Which is kind
of funny. It would be better to maintain a separate uci default upgrade
script for each package. So uci default tasks that are no longer needed
can simply be deleted without having to watch and test the whole scirpt.

- The script is also not so easy to maintain, because the code is not
easy to read.

Signed-off-by: Florian Eckert <redacted>