Remove MFA and blocking functions hints for codelab

[rachelmyers]

This removes the commented out implementation for MFA and the hints at implementation in the blocking function.

The commented out implementation was the best thing to do for the in-person class, but to make it work as a long-lived codelab, let's stick to the conventions for codelabs of having the implementation they need in the codelab itself. (And in the solutions directory, which I didn't change.)