Simplify ExactlyOnePresentValidator and labels model
What does this MR do and why?
Small change to simplify ExactlyOnePresentValidator. In the past a proc was added to the label model error message to keep it intact, but I think it's fine to change the error message a bit in favor of simplifying this code. I think the generic message is good enough for most models and we can add the translation there.
Validator originally added in #558129 (closed)
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