block: Remove elevator_change()

Since commit 84253394

 ("remove the mg_disk driver") removed the
only caller of elevator_change(), also remove the elevator_change()
function itself.

Signed-off-by: Bart Van Assche <bart.vanassche@sandisk.com>
Cc: Christoph Hellwig <hch@lst.de>
Cc: Markus Trippelsdorf <markus@trippelsdorf.de>
Signed-off-by: Jens Axboe <axboe@fb.com>