Simplify type cast and remove error handling in protobuf exercise

[randomPoison]

While using .try_into().expect() is the more robust solution, I think it's more complex than we want here. We don't really care about error handling in this exercise, so I think a simple as cast is clearer here. I think we should also remove the logic for checking if there are enough bytes left, since it simplifies things and makes the solution easier to read.

[djmitche]

In other cases where we've removed something that would be likely in "real" Rust, we've gotten more than one PR trying to put it back. I think it would help to add some commentary here to indicate this is omitted deliberately. Maybe that's as simple as "// TODO: Handle invalid lengths"? That would be enough to fend off any PRs, and also provide a jumping-off point for instructors to talk about what might go wrong here (e.g., split_at might panic) and how to handle it.