btrfs: remove a printk from scan_one_device

Dave pointed out that he saw messages from btrfs although there was no
such filesystem on his computers. The automatic device scan is called on
every new blockdevice if the usual distro udev rule set is used. The
printk introduced in 6f60cbd3

 was a remainder from copying
portions of code from btrfs_get_bdev_and_sb which is used under
different conditions and the warning makes sense there.

Reported-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Josef Bacik <jbacik@fusionio.com>