io-wq: remove unused 'user' member of io_wq

Previous patches killed the last user of this, now it's just a dead member
in the struct. Get rid of it.
Signed-off-by: Jens Axboe <axboe@kernel.dk>

fs/io-wq.c

@@ -110,7 +110,6 @@ struct io_wq {
 	io_wq_work_fn *do_work;
 
 	struct task_struct *manager;
-	struct user_struct *user;
 
 	struct io_wq_hash *hash;