feeds.conf: drop SIDN feed
According to Turris Survey, which is opt-in, I don't see that any routers are using this feed. There are just a few packages related to SIDN Valibox [1].
It will also save our HW resources (CPU, HDD time) and builds will take less time than they should.
If someone wants to use any of these packages, they can compile it by themselves.
[1] https://github.com/SIDN/sidn_openwrt_pkgs
Fixes: #343 (closed)