[BLOCK] elevator: elv_latter/former_request update

With generic dispatch queue update, implicit former/latter request
handling using rq->queuelist.prev/next doesn't work as expected
anymore.  Also, the only iosched dependent on this feature was
noop-iosched and it has been reimplemented to have its own
latter/former methods.  This patch removes implicit former/latter
handling.

Signed-off-by: Tejun Heo <htejun@gmail.com>
Signed-off-by: Jens Axboe <axboe@suse.de>