Remove `jsonData` argument from `image.StoreImage`

The argument specified the json data to save to disk when registering
a new image into the image graph. If it is nil, then the given image
is serialized to json and that is written by default. This default
behavior is sufficient if the given image was originally deserialzed
from this jsonData to begin with which has always been the case.

Signed-off-by: Josh Hawn <josh.hawn@docker.com> (github: jlhawn)
@jlhawn why not `return encoder.Encode(img)` ? :)