[RFC] Remove support for creating rules without an stylesheet

We currently have an api which allows creating rules without a stylesheet associated with them. I think this API isn't needed/used anymore and could be removed.

I believe this adds only more complexity to some of the code because we always need to consider that a rule might not have a StyleSheet.

The issue names no files, tests, or entry points. First locate the API that creates rules without an associated StyleSheet and trace its callers and rule-handling paths. Done means the unsupported API is removed and the code no longer needs to account for rules without a StyleSheet.

Written by the indexing model from the issue text.