[TRIVIAL PATCH] Kill blk_congestion_wait() stub for !CONFIG_BLOCK

blk_congestion_wait() doesn't exist anymore, but there's still a stub
in blkdev.h for the !CONFIG_BLOCK case.  Kill it.

Signed-off-by: Benjamin Gilbert <bgilbert@cs.cmu.edu>
Signed-off-by: Jens Axboe <jens.axboe@oracle.com>