remove the upgrade shell

to be made into a separate application.

The problems with the existing upgrade shell are quite numerous - at the top of my own personal list: it doesn't work at all upgrading plugins; another near-the-top problem being it's very problematic to upgrade the app that's running the upgrade process - i.e. it can only be used to upgrade _something else_.

The current intention is to merge/combine [my own rewrite](https://github.com/AD7six/cakephp/compare/cakephp:3.0...3.0-remove-upgrade), [dereuromark's standalone plugin](https://github.com/dereuromark/upgrade) and produce an officially supported standalone 3.x application.

This will:

* remove a shell which is only of "punctual" use from the core
* prevent testing the upgrade shell from filling the core with needless files/weight
* allow the upgrade shell to progress at a different pace to CakePHP itself (don't need to wait for a cakephp/cakephp release to add some new upgrade step)