block/rbd: drop qemu_rbd_refresh_limits

librbd supports 1 byte alignment for all aio operations.

Currently, there is no API call to query limits from the Ceph
ObjectStore backend.  So drop the bdrv_refresh_limits completely
until there is such an API call.

Signed-off-by: Peter Lieven <pl@kamp.de>
Reviewed-by: Ilya Dryomov <idryomov@gmail.com>
Message-Id: <20210702172356.11574-7-idryomov@gmail.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>