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>

diff --cc block/blk.h
Simple merge