block: remove dead func declaration

__generic_unplug_device() function is removed with commit
7eaceaccab5f40bbfda044629a6298616aeaed50, which forgot to
remove the declaration at meantime. Here remove it.

Signed-off-by: Yuanhan Liu <yuanhan.liu@linux.intel.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

block/blk.h

@@ -36,7 +36,6 @@ bool __blk_end_bidi_request(struct request *rq, int error,
 void blk_rq_timed_out_timer(unsigned long data);
 void blk_delete_timer(struct request *);
 void blk_add_timer(struct request *);
-void __generic_unplug_device(struct request_queue *);
 
 /*
  * Internal atomic flags for request handling