io_uring: Delete useless variable ‘id’ in io_prep_async_work

Fix follow warning:
fs/io_uring.c:1523:22: warning: variable ‘id’ set but not used
[-Wunused-but-set-variable]
  struct io_identity *id;
                        ^~
Reported-by: NHulk Robot <hulkci@huawei.com>
Signed-off-by: NYe Bin <yebin10@huawei.com>
Signed-off-by: NJens Axboe <axboe@kernel.dk>

fs/io_uring.c

@@ -1523,10 +1523,8 @@ static void io_prep_async_work(struct io_kiocb *req)
 {
 	const struct io_op_def *def = &io_op_defs[req->opcode];
 	struct io_ring_ctx *ctx = req->ctx;
-	struct io_identity *id;
 
 	io_req_init_async(req);
-	id = req->work.identity;
 
 	if (req->flags & REQ_F_FORCE_ASYNC)
 		req->work.flags |= IO_WQ_WORK_CONCURRENT;