mtd: nandsim: drop null test before destroy functions

Remove unneeded NULL test.

The semantic patch that makes this change is as follows:
(http://coccinelle.lip6.fr/)

// <smpl>
@@ expression x; @@
-if (x != NULL)
  \(kmem_cache_destroy\|mempool_destroy\|dma_pool_destroy\)(x);
// </smpl>
Signed-off-by: NJulia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: NBrian Norris <computersforpeace@gmail.com>

drivers/mtd/nand/nandsim.c

@@ -649,7 +649,6 @@ static void free_device(struct nandsim *ns)
 				kmem_cache_free(ns->nand_pages_slab,
 						ns->pages[i].byte);
 		}
-		if (ns->nand_pages_slab)
 		kmem_cache_destroy(ns->nand_pages_slab);
 		vfree(ns->pages);
 	}