Retire `url_operations`

[mih]

git-annex v10.20251215 now supports TRANSFER-REDIRECT-URL RETRIEVE {key} {url} as part of the special remote protocol. This makes it possible to let special remote implementation instruct git-annex to perform downloads via any supported URL. This should allow for most custom code to be removed/replaced.

https://git-annex.branchable.com/todo/Special_remote_redirect_to_URL/

I suspect that we won't need to maintain url_operations as a wonderful consequence.