Continue removing BlueGene code from sinfo.

As done previously, this can be removed as it always returns zero:
select_g_select_nodeinfo_get(node_ptr->select_nodeinfo,
                             SELECT_NODEDATA_SUBCNT,
                             NODE_STATE_ERROR,
                             &err_cpus);

Simplify the resulting code, and remove CLUSTER_FLAG_BG code as well.