block: Drop unused bdrv_round_sectors_to_clusters()

Now that the last user [mirror_iteration()] has converted to using
bytes, we no longer need a function to round sectors to clusters.

Signed-off-by: Eric Blake <eblake@redhat.com>
Reviewed-by: John Snow <jsnow@redhat.com>
Reviewed-by: Jeff Cody <jcody@redhat.com>
Reviewed-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>