drbd: Remove an unused field in struct drbd_device

'next_barrier_nr' is not used in this driver. Remove it.

It was already part of the original commit b411b363 ("The DRBD driver")
Apparently, it has never been used.
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/d5322ef88d1d6f544963ee277cb0b427da8dceef.1724602922.git.christophe.jaillet@wanadoo.frSigned-off-by: Jens Axboe <axboe@kernel.dk>

drivers/block/drbd/drbd_int.h

@@ -860,7 +860,6 @@ struct drbd_device {
 	struct list_head read_ee;   /* [RS]P_DATA_REQUEST being read */
 	struct list_head net_ee;    /* zero-copy network send in progress */
 
-	int next_barrier_nr;
 	struct list_head resync_reads;
 	atomic_t pp_in_use;		/* allocated from page pool */
 	atomic_t pp_in_use_by_net;	/* sendpage()d, still referenced by tcp */