'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>