mtd: rawnand: nandsim: Free the partition names in ns_free()

ns_free() is the helper that is called symmetrically to ns_init() and
so should free the same objects, including the partition names.

Now, callers of ns_free() do not need to free this partition names
themselves.

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-11-miquel.raynal@bootlin.com