De-duplicate session flash features.

With a proper FlashComponent and FlashHelper we don't need to have a duplicated set of features in the Session object. This change also unifies the keys SessionComponent/SessionHelper and FlashComponent/FlashHelper use so no matter which interface you use, everything plays nicely together.

Refs cakephp/docs#1430
I was thinking we should keep it around a little bit longer in case people are upgrading. It is also used very frequently in tutorials that exist in the wild. We could have it trigger warnings up until the RC releases.