block/drbd: delete invalid function drbd_md_mark_dirty_

We deleted last_md_mark_dirty long ago, this function no longer needs to
exist, delete it, otherwise a compilation error will occur when DEBUG is
opened.

Fixes: ac0acb9e ("drbd: use drbd_device_post_work() in more place")
Signed-off-by: Jackie Liu <liuyun01@kylinos.cn>
Signed-off-by: Jens Axboe <axboe@kernel.dk>