Remove 'images --tree' and 'images --viz'

* Remove the flag 'docker images --tree'
* Remove the flag 'docker images --viz'

Thse are nice-to-have features and don't need to be in the core client.
They can easily be added as an external project.

This is part of an ongoing effort to keep the core of Docker as small
and simple as possible.

Docker-DCO-1.1-Signed-off-by: Solomon Hykes <solomon@docker.com> (github: shykes)