ruff

c30e80a3 - ruff_db: delete most of the old diagnostic code

ruff_db: delete most of the old diagnostic code We do keep around `OldSecondaryDiagnosticMessage`, since that's part of the Red Knot `InferContext` API. But it's a rather simple type, and we'll be able to delete it entirely once `InferContext` exposes the new `Diagnostic` type directly. Since we aren't consuming `OldSecondaryDiagnosticMessage` any more, we can now accept a slice instead of a vec. (Thanks Clippy.)