Consider to drop SIDN feed

SIDN feed: https://github.com/SIDN/sidn_openwrt_pkgs

• It seems that this feed is no longer active.
  • Pull requests are taking so long to be reviewed and merged
• It does not seems that many users use packages from this feed
• It will save our HW resources, and builds will take less time.

It was added by this commit 67fa9e5f and there is no commit message, why do we need to have it and ship it within Turris OS. If these packages are required by a few people, they can compile it by themselves.