block: Drop leftover references to RQF_SORTED

Commit a1ce35fa ("block: remove dead
elevator code") removed all users of RQF_SORTED. However it is still
defined, and there is one reference left to it (which in effect is
dead code). Clear it all up.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Jens Axboe <axboe@kernel.dk>
Cc: Ming Lei <ming.lei@redhat.com>
Cc: Omar Sandoval <osandov@fb.com>
Cc: Hannes Reinecke <hare@suse.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>