How to fix Cydia “failed to fetch” errors by removing the UltraSn0w repo

One of the more common error messages that new Cydia users will come across is the error that says Failed to fetch [...] HTTP/1.1 404 Not Found. This is a typical 404 message that you get when you visit a webpage location that no longer exists. Normally, this wouldn’t be a big deal, but it causes an error message to appear in Cydia each time you refresh your list of packages and it attempts to pull data from the non-existent UltraSn0w repo.
You’ll receive an error that states: Some index files failed to download, they have been ignored, or old ones used instead. Yeah, so no big deal, but it’s still a huge annoyance, because this interrupts the stream of refreshing the list of Cydia packages. The fact is that you’ll probably never need to use UltraSn0w, and even if you did, you can always add the repo back to Cydia manually.