unregister_blkdev(): delete redundant message

No need to warn unregister_blkdev() failure by caller.  (The previous patch
makes unregister_blkdev() print error message in error case)

Acked-by: Grant Likely <grant.likely@secretlab.ca>
Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>