Allow experiment classes to not be namespaced

What does this MR do and why?

It removes app/experiments from the rule as defined in #321982. The reason is because we don't have an example of an experiment that is namespaced, so all have thus far circumvented it manually, which is a bit frustrating to do. I would instead like to solve this problem by requiring experiment classes to tell us more about which team they're for, what feature flag they're using, and other details like that.

The first step though, is acknowledging that we currently don't use namespaces to do this, but that we do want to help surface more about experiments to other engineers.

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