mtd: rawnand: nandsim: Reorganize ns_cleanup_module()

Reorganize ns_cleanup_module() to fit the reworked exit path of
ns_init_module().

There is no need for a ns_free_lists() function anymore, so drop it.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-18-miquel.raynal@bootlin.com