staging: line6: drop variax model_data field

The sysfs attrs to access the model dump have been removed so it's safe
to drop the model_data field.  The next step will be to simplify the
startup process since we no longer need to fetch this data via dump
requests, but that will be done in a later patch.

Signed-off-by: Stefan Hajnoczi <stefanha@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>