mtd: nandsim: remove unused ns->geom.oobshift

ns->geom.oobshift holds bits number in OOB size, but OOB size is not
always power of two.  So it is useless and it actually isn't used in
this driver except for just printing the value at module loading.

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>