mtd: nandsim: remove unused code

Remove the leftover from commit 831d316b
("mtd: nandsim: remove autoincrement code").

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>