In Review Board 3.0, we officially marked many classes and methods for

deprecation, stating they'd be removed in Review Board 4.0. This change

completes that deprecation process, completely removing them all from

this release.
In many cases, this will trigger import errors in any extensions that

attempt to use these. In a few cases, the breakages will be due to the

wrong methods being implemented by subclasses. Each one, though, would

have resulted in a deprecation warning when run on 3.0.
These will be documented in the release notes.

Unit tests pass.