rbd: kill rbd_image_header::{crypt_type,comp_type}

Image format 1 is deprecated and format 2 doesn't have these.  Also,
__rbd_dev_create() takes care of zeroing (or otherwise initializing)
format 2 specific fields.

Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Reviewed-by: Jason Dillaman <dillaman@redhat.com>