block/drbd: remove unused w_start_resync declaration

w_start_resync has been removed since
commit ac0acb9e ("drbd: use drbd_device_post_work()
in more places"), so remove it.

Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
Acked-by: Christoph Böhmwalder <christoph.boehmwalder@linbit.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>