This removes outputs from yaml, instead you can do
```
moby build -output tar -output qcow2 file.yaml
```
or alternative syntax
```
moby build -output tar,qcow2 file.yaml
```
In future we may change this to be available in a `moby package`
step, but lets try this for now.
Signed-off-by: Justin Cormack <justin.cormack@docker.com>
