mtd: rawnand: nandsim: Remove debugfs entries at unload time

Create a ns_debugfs_remove() helper for that and call it in
ns_cleanup_module().

Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/linux-mtd/20200525085851.17682-7-miquel.raynal@bootlin.com