Deprecate 'docker images --tree' and 'docker images --viz'

As discussed on #docker-dev, these flags can easily be implemented as external tools. We want to keep the Docker core as small as possible. Let's start cutting down the "nice-to-have" features.

* The commands are no longer listed or documented.
* The commands still work but print a deprecation warning.
* The commands should be removed in a future version.