Remove buf.validate.Violations Protobuf message

[bufdev]

We don't have a super-concrete use case for having a buf.validate.Violations message, as opposed to using lists of buf.validate.Violation messages. There's a thought that we may want a repeated message wrapper for RPC error details, but we haven't discovered this just yet, and we want to minimize API surface area as much as possible. We'll look to batch this with other breaking changes pre-v1.0.