UBI: 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: Julia Lawall <Julia.Lawall@lip6.fr>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/attach.c

@@ -1209,9 +1209,7 @@ static void destroy_ai(struct ubi_attach_info *ai)
 		}
 	}
 
-	if (ai->aeb_slab_cache)
 	kmem_cache_destroy(ai->aeb_slab_cache);
-
 	kfree(ai);
 }