Remove a few controller methods.

Remove controller methods that are not frequently used, or are a mismatch
for the layer they are located in.

* Controller::flash() promotes a pattern no one uses anymore.
* Controller::validate() and Controller::validateErrors() both live
  at the wrong level.

Refs #2375
Remove another unused method.

BeforeRedirect filters are much simpler now and this method is no longer
needed.

Refs #2375
Remove leftovers from Controller cleanup.

Refs #2375
Merge pull request #2385 from markstory/3.0-controller-cleanup

3.0 controller cleanup