cfq-iosched: no need to keep track of busy_rt_queues

o Get rid of busy_rt_queues infrastructure. Looks like it is redundant.

o Once an RT queue gets request it will preempt any of the BE or IDLE queues
  immediately. Otherwise this queue will be put on service tree and scheduler
  will anyway select this queue before any of the BE or IDLE queue. Hence
  looks like there is no need to keep track of how many busy RT queues are
  currently on service tree.

Signed-off-by: Vivek Goyal <vgoyal@redhat.com>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>