block: get rid of kblock_schedule_delayed_work()

It was briefly introduced to allow CFQ to to delayed scheduling,
but we ended up removing that feature again. So lets kill the
function and export, and just switch CFQ back to the normal work
schedule since it is now passing in a '0' delay from all call
sites.

Signed-off-by: Jens Axboe <jens.axboe@oracle.com>