bcachefs: Drop bch_write_op->io_wq

This is dead code
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/io.h

@@ -71,7 +71,6 @@ static inline void bch2_write_op_init(struct bch_write_op *op, struct bch_fs *c,
 				      struct bch_io_opts opts)
 {
 	op->c			= c;
-	op->io_wq		= index_update_wq(op);
 	op->flags		= 0;
 	op->written		= 0;
 	op->error		= 0;

fs/bcachefs/io_types.h

@@ -95,7 +95,6 @@ struct bch_write_bio {
 struct bch_write_op {
 	struct closure		cl;
 	struct bch_fs		*c;
-	struct workqueue_struct	*io_wq;
 	u64			start_time;
 
 	unsigned		written; /* sectors */
@@ -111,7 +110,6 @@ struct bch_write_op {
 	struct bch_devs_list	devs_have;
 	u16			target;
 	u16			nonce;
-
 	struct bch_io_opts	opts;
 
 	struct bpos		pos;