rbd: mark block queue as non-rotational

Set QUEUE_FLAG_NONROT.  Following commit b277da0a

 ("block: disable
entropy contributions for nonrot devices") we should also clear
QUEUE_FLAG_ADD_RANDOM, but it's off by default for blk-mq drivers, so
just note it in the comment.

Also remove physical block size assignment - no sense in repeating
defaults that are not going to change.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>