Removed impossible branch of 'if' statement

Scaling is called with the following code:

----------
    cluster = conductor.cluster_update(ctx, cluster,
                                       {"status": "Decommissioning"})
...

    cluster = conductor.cluster_update(ctx, cluster, {"status": "Scaling"})
...
    instances = INFRA.scale_cluster(cluster, node_group_id_map)
----------

Cluster can't be in decommissioning state inside scale_cluster.

Removed code looks like attempt to move cluster to Error state in case if some
nodes were removed. According to comment in Heat engine, our policy is to remove
added node, but do not add removed nodes back (need to update docs?).

So, just removing impossible branch of if statement.

Change-Id: Iafc35807c7b03b8e4a19f2b5775143a5830cee5f
Merge "Removed impossible branch of 'if' statement"