xerox_mfp: do not append slack lines

xerox_mfp devices return a page height in millimetres. We currently convert that to pixels and add "slack" lines if the scanner doesn't return enough lines.

This seems like a good solution intuitively, as the driver passes through as much information as possible to the frontend. But in practice, it forces the user to spend time manually cropping lines from an otherwise-acceptable image.

This patch removes slack data and leaves the image height at -1 (i.e. doesn't guarantee an exact line count).