mtd: ubi: Rmove unused declaration in header file

The definition of ubi_destroy_ai() has been removed since
commit dac6e20 ("UBI: Add fastmap stuff to attach.c").
Remove the empty declaration in header file.

Signed-off-by: Zhang Zekun <zhangzekun11@huawei.com>
Reviewed-by: Zhihao Cheng <chengzhihao1@huawei.com>
Signed-off-by: Richard Weinberger <richard@nod.at>

drivers/mtd/ubi/ubi.h

@@ -831,7 +831,6 @@ void ubi_remove_av(struct ubi_attach_info *ai, struct ubi_ainf_volume *av);
 struct ubi_ainf_peb *ubi_early_get_peb(struct ubi_device *ubi,
 				       struct ubi_attach_info *ai);
 int ubi_attach(struct ubi_device *ubi, int force_scan);
-void ubi_destroy_ai(struct ubi_attach_info *ai);
 
 /* vtbl.c */
 int ubi_change_vtbl_record(struct ubi_device *ubi, int idx,