dvdrw-tools: delete explicit default for backend

Signed-off-by: Steve Kenton <skenton@ou.edu>
Signed-off-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Acked-by: Angelo Compagnucci <angelo.compagnucci@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>